Rear Dormer Loft Conversion Cost

A rear dormer loft conversion is a box-like extension added to the back of a roof to create more head height and usable floor space.

This type of loft conversion is a popular choice for homeowners who want to turn an unused loft into an extra bedroom, home office or family bathroom.

In this guide, we’ll look at rear dormer loft conversion costs, what can affect the final price and what to expect when hiring a loft conversion specialist.

How Much is a Rear Dormer Loft Conversion?

Built onto the rear slope of the roof to extend the habitable space in the loft, in particular to increase the headroom. It’s a popular choice for homeowners as it’s an effective use of the loft when space in the home is limited.

The dormer, with its flat roof, sits on the back of the house The average cost of a rear dormer loft conversion is between £35,000 and £60,000, with many influencing factors affecting the final price, including:

• Size of the dormer
• Property type
• Amount of structural work required
• Whether an ensuite bathroom is included
• Level of internal finish
• Location

A rear dormer extension is more expensive than a simple rooflight conversion but less costly than a hip-to-gable, L-shaped dormer or mansard loft conversion.

Location obviously plays a part in rear dormer prices, with London being more expensive than most other places in the UK.

Rear Dormer Loft Conversion Prices

Each loft conversion is different, but to give you an idea of price ranges and what’s involved in different jobs during the project, check out the table below.

Supply Costs

The supply costs for a rear dormer loft conversion depend on the size of the dormer and the type of materials chosen for the exterior of the build. These costs rest on whether the project is a shell-only job or finished to a fully habitable standard.

Below are the costs for the type of materials used in the construction of a rear dormer conversion.

Timber

A key component for the build, timber used in a loft conversion needs to be structural grade, typically C24-rated softwood. It’s used for the floor joists, rafters, purlins and ridge beams. Costs vary depending on the dimensions of the timber but range from £9 to £23 per linear metre.

Structural Steel

The size of the steel varies according to the specifications for the build, but costs typically range from £50 to £240 per linear metre.

Roofing Materials

A rear dormer has a flat roof and is typically covered using EPDM rubber, torch-on felt, or GRP, also known as fibreglass roofing.

EPDM costs around £10-£15 per square metre and is a highly durable membrane with a 50-year lifespan. GRP is approximately £35-£45 per square metre, providing a seamless, rigid covering.

Traditional torch-on felt utilises multiple layers to create a waterproof, seamless covering for your dormer roof. Underlay costs £45-£65 per 16m roll with mineral cap sheets or top layer priced at about £45-£75 per 8m roll.

Insulation

The type of insulation used in a rear dormer conversion influences how much it costs. Rigid PIR board is typically fitted in ceilings, walls, and floors and costs £15-£60 depending on thickness and brand. Mineral wool insulation costs between £5 and £20 per square metre.

Plasterboard

Standard 12.5mm plasterboard is used for loft conversions, and a 2400mm x 1200mm or 8’ x 4’ sheet costs from £10-£14. Buying in bulk will lower the price.

Dormer Cladding

Dormer cladding protects the walls or ‘cheeks’ of the dormer, as well as providing a decorative finish and improving insulation. The choice of material dictates how much you’ll pay.

Common materials used are timber such as larch, slate, tiles, zinc or composite. Prices range from £35 to £100 per square metre for composite and between £100 and £150 per square metre for zinc.

Windows

Window prices vary according to the size, profile and frame material. Rooflight windows, commonly known by the brand Velux, cost from £350 to £750 and are available as top hung or centre pivot.

Dormer windows are available in uPVC, timber, aluminium or hardwood, and prices range from £800 up to £5,000. If a Juliet balcony is installed, French windows will cost around £500 to £2,500, depending on frame material.

Staircase Materials

Stairs for rear dormer loft conversions can be made from softwood, hardwood or metal, with prices varying accordingly. Softwood is the most common and costs £600-£1,500 on average.

Hardwood stairs can cost as much as £3,000. Bespoke or industrial style stairs may add several thousand to your budget.

Flooring

The choice of the finished flooring material impacts your budget, with homeowners choosing from engineered wooden floors, carpet, vinyl or laminate. Prices range from £25 - £75 per square metre for carpet to £40 - £100 per square metre for engineered wooden flooring.

Sub-floor materials like moisture-resistant chipboard cost £10 - £20 per 2400mm x 600mm sheet.

Electrics

Supply-only costs for 1st and 2nd fix electrics in a rear dormer loft conversion are approximately £1,000-£1,200. However, the choice of 2nd fix fittings can significantly increase this cost if high-end or luxury items are chosen.

Plumbing Materials (where relevant)

Where an ensuite bathroom is included in a loft conversion, the overall cost rises. Sanitary ware, pipework, tiles and heaters cost from £1,000 to £2,500 depending on quality and brand.

Internal Finishes

The level of finish internally is a major contributor to the overall cost of a rear dormer loft conversion. Whether you go standard or premium will be reflected in how much you pay to fit out your new loft.

Additional Costs

Beyond the main and more obvious rear dormer loft conversion cost, there are extra expenses that you need to bear in mind.

Architectural Drawings

The architect's drawings include a site plan, floor plan, sections and elevations, which are used for planning approval, ordering materials and as a guide to construction on site. Architect’s fees range from £2,500 to £7,500.

Structural Calculations

The structural engineer works in collaboration with the architect. They produce the structural calculations which ensure that the property can withstand the extra load of the new loft conversion. The cost of a structural engineer is between £750 and £2,000.

Planning Application Fees

When submitting a planning application for a loft conversion or other type of build, there is a fee to pay. For 2026, this fee is £548 and can be checked via the Planning Portal.

Building Control Charges

At various stages of construction, building control will ask to inspect certain parts of the build. Some private companies carry out these inspections on behalf of the local authority. Charges can vary, but an average cost is £800 to £1,500.

Party Wall Surveyor Fees

If your property is a terraced home or semi-detached, you most likely share a party wall with your neighbour. As a loft conversion impacts this wall, you need to serve them notice of your intention to build off this wall. A party wall surveyor may be needed if your neighbour doesn’t respond to the notice and a survey is required. Fees vary but generally are between £900 and £2,500.

Scaffolding

The cost of scaffolding is impacted by the extent of the scaffold, i.e., how many sides there are, location and the duration of the hire. For a rear dormer, the price you pay is likely to be between £1,800 and £3,000. Prices in London are 20-30% higher than the rest of the country.

Skip Hire

A skip is essential for a loft conversion as the work generates a lot of waste. Costs depend on the size of the skip, duration of hire and whether it’s going to be on your driveway or the public highway. If the latter, you’ll also have to pay for a permit, adding an extra £30-£70 to your hire costs. An 8-yard skip costs £200 - £350 per week.

Waste Removal

You’ll clearly need more than one skip for the duration of the build. How often the skip is swapped will depend on the waste volume generated by construction. A rear dormer typically goes through 2-3 skips per project.

Roof Strengthening

Strengthening the roof with steel, new timbers and necessary alterations according to the structural engineer’s calculations costs £5000-£11,000.

Moving Pipework or Tanks

Older properties may still have cold water tanks in the loft, which need to be moved. Costs vary but are around £200-£350. If pipework also needs to be relocated, the price will depend on the extent and complexity of the work. Many homeowners choose this time to upgrade their boiler to a combi or unvented hot water cylinder.

Electrical Upgrades

Upgrading the electrics as part of the rear dormer construction can cost from £1,000 to £3,500. With additional electrics, it is often necessary to replace the consumer, which costs around £500.

Plumbing for an Ensuite

With enough space, many homeowners choose to add an ensuite when converting their loft, and this adds to the overall price by about £2,000 to £5,000, not including the sanitary ware. The choice of bathroom suite, cheap and basic, mid-range or high-end, will add to the cost accordingly.

Decorating

Some homeowners are happy to DIY the decorating, while others prefer to let a professional handle it, which will cost around £150 per square metre. This price includes all walls, ceilings and woodwork.

Some loft conversions may require remedial roof work or floor strengthening.

Remedial Roof Rork

Before the dormer is built, rafters may need to be strengthened by doubling them up with new timber, which is also called sistering. It ensures the rafters can handle the increased load. RSJs are fitted, and new floor joists are added. The work can cost between £1,200 and £3,000.

Floor Strengthening

The joists on the loft floor are actually the ceiling joists for the floor below; they were never designed to hold up another storey. Therefore, the floor of the loft needs to be reinforced with C16 or C24 structural grade timber joists, running parallel to the originals. C16 costs £1.30-£4 per metre and C24 costs £3 - £10 per metre.

Labour Costs and Timescales

Two of the most pressing questions for homeowners when undertaking a loft conversion are the costs and the timescales. Let’s take a look at both.

Labour

Labour costs comprise a large part of rear dormer prices as multiple trades are needed to complete the work. The following are the tradespeople who work on a loft conversion, along with the national average day rate for their profession.

• Carpenter: £240 - £360
• Roofer: £280 - £360
• Plasterer: £150 - £250
• Bricklayer: £240 - £320
• Electrician: £300 - £500
• Snagman: £180 - £230

Some optional trades are:

• Plumber: £320 - £480
• Tiler: £200 - £350
• Decorator: £250 - £350

A scaffolder will charge £280 - £320 per day, but most loft conversion specialists work with a local scaffolding company and agree prices in advance with them. Scaffolding is usually included in your quote.

Labour prices in London and the southeast are typically 15-25% higher than the rest of the UK.

Timescales

The timescale for a rear dormer loft conversion depends on a number of factors.

• Dormer size: the bigger the dormer, the longer it takes to build
• Complexity of roof alterations - issues like a truss roof are more complicated to work on
• Site access: difficult access for vehicles or machinery hinders the build and deliveries, increasing the need for manual labour and slowing the pace of work
• Weather conditions: a loft conversion is heavily weather-dependent; storms and high winds can delay the project
• Project includes ensuite: adding an ensuite will increase the timescale

The timescale for a shell-only rear dormer is around 6 - 8 weeks, while a fully finished dormer takes 10 -12 weeks.

Cost Factors for a Rear Dormer Loft Conversion

The following are the main cost factors for a rear dormer conversion. Understanding these costs allows you to better compare the quotes you receive and lets you make an informed decision.

Property Size

Potentially the most obvious cost factor is the size of the property. A large detached home will accommodate a bigger rear dormer, while a terraced house is more restricted. More materials and labour are clearly needed for a large dormer build and for fitting out internally.

Existing Roof Structure

Most properties have a traditional cut roof, but some homes have truss roofs, which complicate a loft conversion. A lot more work is needed as the trusses have to be replaced with an A-frame for structural support. The extra work and materials add to the overall price.

Dormer Size

There are various sizes of rear dormers, including full-width dormers, L-shape dormers and small box dormers, often only big enough for a single window.

Under permitted development rights, terraced homes are restricted to adding 40 cubic metres of new space. Detached and semi-detached properties have a 50 cubic metre allowance. This can also determine the size of the dormer.

Amount of Structural Alteration

The majority of structural alteration for a rear dormer involves the necessary installation of new joists and steel beams to support the increased load. Truss roofs require more extensive work to make a loft conversion feasible, adding to the cost.

Steelwork Requirement

The length and size of the steel beam required are determined by the structural engineer, and this varies from one job to another. Three steels are typical, with two supporting the floor and one supporting the ridge. The dimensions of each steel will impact the cost.

Staircase Design

The most common staircase design for a standard rear dormer is softwood winder stairs. Depending on the layout, a straight staircase may be possible. More complex or bespoke designs will increase your costs.

Number and Type of Windows

The number and type of windows in your dormer conversion will impact the price you pay.

It’s not just increasing the number of windows that pushes costs up, but the labour and materials involved in creating more apertures and fitting the windows.

Frame choice is the biggest cost decision with uPVC, timber, aluminium, composite or steel to choose from. uPVC is the most affordable, with steel or hardwood being the most expensive.

Insulation Levels

Building Regulations stipulate a minimum U-value of 0.15–0.18 W/m²K for insulation, which is typically achieved with 270mm of mineral wool or 150mm of PIR board. Thermal plasterboard is sometimes used to reduce thermal bridging, and this costs up to 4 times as much as standard plasterboard.

A warm roof, where insulation is installed over or between rafters, is recommended for most loft conversions, adding to the insulation costs.

Whether an Ensuite is Included

Including an ensuite in your loft conversion typically adds between 10 - 15% to the budget. The final choice of design and quality of fittings significantly affects how much you pay, with high-end sanitary ware costing much more than standard items.

Interior Finish standard

The level of finish internally will affect the overall price of the loft conversion project. Choosing from basic, mid-range or high-end finishes impacts your budget accordingly.

Property location

Where you live in the UK affects material costs, labour rates and even parking charges. All of these add up. Homeowners in London and the southeast are aware of the increased costs they face for improvement projects, which can be 15 - 25% higher than the rest of the country.

Ease of access

Poor access to the site can have surprisingly expensive repercussions. Deliveries may have to be manually unloaded, skips may have to be smaller than desired, and scaffolders may charge more when erecting or dismantling the scaffold. Trades may have to pay to park elsewhere, and a steel beam delivery can become very expensive.

With all of these cost factors to consider, it’s easier to see why not all quotes are the same and highlights the importance of comparing them carefully.

Building Regulations Related to a Rear Dormer Loft Conversion

All loft conversions need Building Regulations approval, and inspections are done at key stages of the build to ensure that these regulations are being adhered to. The local authority may send its own inspectors, or it may contract the work out to private companies.

Structural Safety

The first inspection is done to check that the steel beams installed are capable of supporting the new floors and ridge. Part A of the Building Regulations covers structural elements of the build, including the RSJs.

Fire Safety

Part B, or Approved Document B, concerns fire safety, including means of escape, fire spread and structural protection. It covers fire doors , escape windows, plasterboard thickness and smoke detectors.

Stairs

The design of the staircase, including headroom, pitch and step dimensions, is covered by Part K of the Building Regulations.

Insulation

Insulation requirements for lofts are stipulated by Approved Document L as part of energy efficiency standards. A minimum U-value of 0.15–0.18 W/m²K is required from insulation laid in the loft.

Ventilation

Approved Document F requires adequate ventilation within the loft space, including the use of trickle vents, extractor fans and openable windows. This is designed to ensure good air quality and reduce the formation of damp.

Sound Resistance

Part E of the Building Regulations relates to the transmission of sound and the need to include sound insulation to reduce noise, such as footfall. Typically, this means introducing at least 100mm of insulation between floor joists.

Means of Escape

As part of Approved Document B, the means of escape includes requirements for the loft staircase to lead to a protected route, such as a hallway or landing that in turn exits via an external door. Escape windows must be easy to open and not require a key.

Permitted Development vs Planning Permission

Permitted development (PD) rights refer to the homeowner's ability to create changes within the property that don’t require planning permission.

Many loft conversions fall under PD rights. Terraced homes can add up to 40 cubic metres of new space, and semi or fully detached homes are allowed 50 cubic metres without needing planning permission. If your dormer exceeds this limit, planning permission is needed.

Homes in conservation areas typically need planning permission to add a rear dormer.

Party Wall Matters

A party wall agreement between neighbours is needed if a shared wall is being used as part of the loft conversion. Usually, this applies to terrace or semi-detached homes.

Typical works on the party wall include cutting pockets into the wall to rest the steel beams on, building up the party wall as part of the dormer or removing a shared chimney breast.

Types of Rear Dormer Loft Conversion

There are a few different types of rear dormers, all of which create extra headroom and floor space in the loft.

Flat Roof Rear Dormer

As the name suggests, this type of rear dormer has a flat roof. However, that doesn’t mean it’s perfectly level; it still needs a slight gradient to allow rain runoff. The (almost) flat roof gives the dormer a box-like appearance with neat lines, allowing it to blend into the house.

As the most popular type of dormer, it is straightforward to build, and prices range from £40,000 to £55,000 on average.

Pitched-roof Rear Dormer

A pitched roof rear dormer often looks less like an added-on extension and more like an integral part of the original property. By matching the pitch on the dormer with that of the main roof, a more blended appearance is achieved, as well as added head height.

It is a more complex build than a flat roof, but most loft conversion specialists are capable of creating this type of dormer. Prices range from £45,000 to £65,000.

Full-width Rear Dormer

As the name suggests, a full-width rear dormer uses the whole expanse of the roof rather than being stepped-in as with other dormers. This provides extra floor space as well as increased head height and an abundance of natural light. In some instances, a slight step-in of 300mm is required for planning reasons.

A full-width rear dormer is built in the same way as a standard-width dormer, with adjustments made to support the structure across the entire span of the roof. Costs range from £45,000 to £75,000.

Gable Dormers

A gable dormer is a small traditional dormer with a mini-pitched roof. It’s sometimes referred to as a doghouse dormer due to its appearance.

These small dormers provide natural light and usable head height where a full dormer is not possible and are most often seen on the front of houses. For this reason, they almost always need planning permission.

Despite their diminutive size, a gable dormer is more complex to build than a rear dormer. The joinery is more involved with the pitched roof, valleys and ridge boards. Costs vary but average around £800 to £1,200 per gable dormer.

Alternative Types of Rear Dormer Loft Conversion

The alternatives to a rear dormer are a rooflight conversion, hip-to-gable, mansard or L-shape dormer and a full home extension. However, not every option is suitable for every property.

Rooflight Loft Conversions

A rooflight loft conversion is the most affordable as there are no significant alterations to the roofline. Average costs are between £30,000 and £40,000, and disruption is minimal.

No additional head height is created, so the roof slope needs to be steep enough to make the loft usable as habitable space. A rooflight loft conversion can be done under permitted development, in most cases.

Hip-to-gable Conversions

A hip-to-gable loft conversion creates significant head height as the side slope of the roof is lifted vertically. Semi-detached and detached homes can create an extra 50 cubic metres of added room under permitted development with this type of conversion.

There is a lot of disruption with a hip-to-gable as a large part of the roof is removed, expect a lot of noise and dust. Prices range from £45,000 to £60,000.

Mansard Loft Conversions

A mansard loft conversion doesn’t work for every property, but with the right house, it can create a vast amount of habitable room. Head height and floor space are significantly increased.

Costs range from £60,000 to £80,000 or more, especially in London, and they almost always need planning permission due to the extent of the structural alterations. This means a lot of disruption with lots of noise, dust and possible loss of services.

L-shaped Dormer Conversions

An L-shaped dormer is possible when a property features a rear addition or outrigger. The L-shape is formed when a dormer is built onto the rear of the house, and another is built on the addition. It creates enough space for two bedrooms and an ensuite or a large master bedroom with a walk-in wardrobe and ensuite.

Cost is around £50,000 to £90,000, and the build is complex and highly disruptive, with the need to join two dormers together. If the total space achieved doesn’t exceed the allowances of 40 cubic metres for terrace homes or 50 cubic metres for semi-detached or detached, planning is not needed unless the property is in a conservation area.

Full Home Extensions

A full home extension may comprise a wraparound or multi-storey extension or a combination of a loft conversion and rear extension. Under permitted development rights, you can add 40 - 50 cubic metres, depending on the type of property you have. This includes extensions or loft conversions. If you exceed this limit, you need to apply for planning permission.

Costs range from £1,800 to £5,000 per square metre, depending on location and level of finish. The extent of the works means that disruption is high with noise, dust and potential loss of services.

Hiring Contractors Checklist for a Rear Dormer Loft Conversion

Before hiring contractors for a rear dormer loft conversion, it’s helpful to think about the kind of questions you might want to ask.

Checking Experience with Loft Conversions and Dormer Builds

Unless you’re hiring a loft conversion specialist, it’s important to ask if the contractor has experience with loft conversions and dormer builds in particular. Loft conversions have their own unique requirements, specifications and potential issues which a general builder may not be familiar with.

Asking to See Previous Similar Projects

A professional contractor or loft conversion company will have no problem referring you to previous clients who are willing to show you around their completed loft. It often helps when you find it difficult to visualise the final result, and gives you a chance to get feedback on the contractor.

Confirming Who Handles Drawings and Calculations

Many loft specialists work closely with independent architects and structural engineers or have their own in-house team. Check that either of these is the case to ensure that there is a qualified professional taking care of the drawings and calculations.

Checking Whether Planning and Building Control are Included

Ask if the submission of your planning application and building control liaison is included in your quote. It’s important that you request detailed quotes from several contractors so you can compare like-for-like.

Understanding What is Covered in the Quote

If you’re unsure what is covered in the quote or the language is overly technical or vague, it’s important to ask questions and clarify the information. It’s better to ask than assume and find yourself with an expense you didn’t anticipate.

Confirming the Project Timescale

Loft conversion companies have experience with a variety of loft conversions and can give you a fairly accurate timeline for completion. You can confirm their intended completion date with them, but be aware that roof work in particular is subject to weather-related delays. Once the loft is weather-tight, this is not usually a problem.

Checking Insurance

Ask for proof of insurance cover, in particular public liability insurance, employers' liability, professional indemnity and contractor’s all-risk insurance. Many companies will supply copies of this proof with your quotation package,e but if they don’t, ask for it.

Asking About Guarantees

Loft specialists typically provide a 10-year structural guarantee covering the main elements of the build. Some may offer insurance-backed guarantees which cover any faults if the company ceases trading.

Comparing Several Detailed Quotes

For any large project, you should request several detailed quotes so that you can compare them, like-for-like. This gives you an idea of the average cost of the work and helps to negotiate prices with your preferred contractor.

Removing a Rear Dormer Loft Conversion

A rear dormer may need to be removed or altered if it’s old and no longer complies with Building Regulations, is badly built and structurally unsound or leaking.

Corrective Works Where an Old Dormer is Defective

A dormer that is defective is unsafe and needs to be made safe again, if possible. Corrective works should always be done by a qualified professional to ensure they meet current building standards.

Leaking

A leaking roof may necessitate taking down the dormer if there is extensive water damage. Timbers may be rotten, and insulation that is wet offers no thermal value. If mould or mildew are present due to damp, this can be a health hazard. A new roof then needs to be installed.

Unauthorised

A dormer that has been built illegally, that is, without planning permission or exceeding the allowance of permitted development, may be removed. However, if a dormer has been in continuous use for more than four years, even if unauthorised, it may not necessarily be removed. From April 2024, this rule increased to ten years of continuous use.

Poorly Insulated

A poorly insulated loft conversion can be improved, but remedial works can be expensive and disruptive. The old insulation in floors, walls and ceilings has to be removed and replaced. This means removing plasterboard, floorboards and decorative finishes to complete the work, not to mention the cost of new insulation and waste removal.

Structurally Unsound

A structurally unsound dormer should be removed if it cannot be safely strengthened. The local authority may issue an enforcement notice to repair or remove the dormer if they think it poses a danger.

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