Side Dormer Loft Conversion Cost

Building a side dormer loft conversion involves putting a box-shaped extension on the side slope or hip of a roof. It allows increased headroom and more habitable space in the loft by extending outwards. Homeowners may choose a side dormer as a cheaper alternative to a hip-to-gable conversion.

With the additional head height, it’s possible to create an extra bedroom, office, dressing room or bathroom in the loft. This is a popular and affordable way to extend your living space, with costs depending on loft size, structural work needed, level of finish and your location. Let’s take a look at what’s involved.

How Much Is a Side Dormer Loft Conversion?

The cost of a side dormer loft conversion depends on a number of factors, not least of which is the specification. A basic conversion costs around £35,000, a mid-range finish will be more costly at around £40,000, while a premium finish can set you back around £50,000.

Influencing factors are the size of the dormer, whether an ensuite is included, your location and the specification of the final finish. This final factor can mean choosing hardwood flooring over carpet or high-spec sanitary ware over standard items.

Other choices that you make regarding the build, such as having a bespoke staircase, upgraded insulation or high-end finishes throughout, will increase the cost of a side dormer loft conversion. Location also affects cost, with labour rates being higher in London and the southeast.

Side Dormer Loft Conversion Prices

Although costs vary depending on size, location and job specification, it’s helpful to have a general idea of prices, timescales and what different tasks entail as part of the loft conversion.

Supply Costs for a Side Dormer Loft Conversion

Side dormer loft conversion costs vary depending on the size of the loft and the quality of materials chosen. For example, you can choose standard, premium or bespoke items according to your taste and budget.

Naturally, more high-end or bespoke items will increase the final price of the build.

Timber

Structural grade timber is used when building a dormer, typically C24 - rated softwood. It’s needed for floor joists, rafters, purlins and structural ridge beams and therefore comes in different sizes. Prices vary from £9 to £13 per linear metre.

Structural Steel

Most dormers require three structural steels: two for the new loft floor and one for the ridge. The dimensions of the beams are calculated by the structural engineer based on the load and the condition of your property. Costs range from £50 to £240 per linear metre.

Insulation

PIR rigid board insulation is most often used in the walls, ceiling and floors of a loft conversion. Prices range from £15 to £60 on average, depending on the board’s thickness. Mineral wool may also be used and costs around £5-£20 per square metre.

Dormer Cladding

Dormer sides or ‘cheeks’ are covered with cladding options to protect them from the weather, help with insulation and provide a decorative finish. Among the choices are uPVC, timber, tiles, slate, zinc and composite.

Prices range from £12 to £44 per square metre for larch cladding, depending on quality, i.e., British or Siberian. Composite cladding prices are £35 to £100 per square metre.

Membranes

The roofing material for a side dormer will be flat roof specific. Choices are traditional torch-on felt, EPDM rubber or GRP (fibreglass). Prices range from £45-£75 for 8m rolls of felt top coat and £45-£65 for 16m of underlay, £10-£15 per square metre for EPDM and £35-£45 per square metre for GRP.

Plasterboard

The standard size plasterboard used in a loft conversion is 12.5mm, which costs £10-£14 per 2400mm x 1200mm sheet. Larger quantities should mean a lower price per unit.

Windows

A side dormer loft conversion can include rooflights or Velux windows, dormer windows or even French doors with a Juliet balcony. The size, number and frame material all dictate how much you pay.

Rooflights cost on average £350-£750 and can be either top hung or centre pivot. The latter gives more headroom and is slightly more expensive.

Dormer windows may be casement, sash or tilt and turn. Prices vary from £800 to over £5,000. French doors can cost up to £2,500 for aluminium frames, but more affordable uPVC doors cost between £500 and £900.

Staircase Materials

Most stairs for a loft conversion come ready-made to the site and are fitted by the carpenter. The material they’re made from determines their cost.

Softwood is the most common and affordable choice, ranging from £600 to £1,500. Hardwood is pricier, with stairs costing between £1,500 and £3,000. For an industrial style finish, metal stairs will set you back £2,000-£3,000.

Flooring

The finished flooring choice has a significant impact on cost. Homeowners can choose from carpet, wooden flooring, laminate, LVT or engineered wood. Prices range from £25-£75 per square metre for carpet and £40-£100 per square metre for engineered floors.

A sub-floor is necessary for a loft conversion. Moisture-resistant chipboard used for this purpose costs £10-£20 per 2400mm x 600mm sheet.

Fixings

A lot of fittings are used during a loft conversion. From joist hangers to plasterboard screws, hinges to insulation fixings, there’s a lot to consider. Prices will depend on the size of the project, but expect supply costs to be around £800 to £1,500.

Joinery Products

Joinery costs will depend on the choice of doors, skirting boards, mouldings and the stair handrail, as well as on the size of the loft conversion. Internal doors can cost between £50 and £200, depending on the material, e.g., hollow doors versus solid timber.

Average skirting board prices are £5-£15 per linear metre, depending on material and profile. MDF skirting is cheaper than solid wood. Stair handrails cost between £16 and £60 per metre, depending on whether it’s softwood or hardwood.

If a side dormer loft conversion includes an ensuite bathroom, there will be extra costs. The average supply-only material costs, not including labour, are as follows:

Sanitaryware

Space-saving sanitary ware sets cost between £400 and £800 for a toilet, corner shower enclosure & tray and vanity unit. Branded or high-end options can cost from £800 to £1,500.

Pipework & Fittings

Supply costs for pipes, fittings and waste range from £300-£800 depending on whether push-fit plastic or copper is used. The distance to the soil stack also impacts the price, as longer pipe runs increase costs.

Tiling

Tiles for an ensuite can be of varying quality, size and material, and as a result, the cost per square metre ranges from £15 to £150. Large format tiles or those made from natural stone are more expensive than smaller formats or those made of ceramic and porcelain.

Waterproofing

A loft bathroom may need to be ‘tanked’, which means waterproofed, costing between £300 and £500 for materials.

Additional Costs of a Side Dormer Loft Conversion

Some additional costs may arise before, during or after the project, which also need to be included in your budget, or at least considered.

Scaffolding

Scaffolding prices are determined by how many sides are needed, your location in the UK and the duration of the hire. For a side dormer, the average cost of scaffolding is around £1,800 to £3,000. Prices in London are typically at the high end of the scale.

Skip Hire

Skip hire is essential when doing a side dormer as the build generates a lot of waste, including old insulation, bricks, roof tiles, old floorboards and timber offcuts. An 8-yard skip is typically used, and this costs between £200 and £350 per week to hire.

Waste Removal

The amount of waste created during a loft conversion means that the skip will need to be swapped two or three times. Builders often use old sheets of construction ply to build up the sides of the skip to fit more inside.

Architectural Drawings

Without the architectural drawings, it would be impossible to correctly and safely build a side dormer. The drawings provide a scaled representation of the build and are used for planning applications, construction guidance and material ordering. Fees for an architect are between £2,500 and £7,500.

Structural Engineer Calculations

Used in conjunction with the architect’s drawings, the structural calculations determine the dimensions of the structural steels, assess loads and ensure Building Regulation compliance. A structural engineer’s fee is between £750 and £2,000.

Planning Application Fees Are Required

If planning permission is required, the cost to submit an application is currently £548. These fees typically change every year, but can be checked via the Planning Portal.

Most side dormers can be constructed under permitted development rights, but properties in conservation areas generally have to apply for planning permission.

Party Wall Costs

When your property shares a party wall, you need to send your neighbour a party wall notice to tell them you are intending to build your loft conversion off this wall. If they don’t respond or object, a party wall surveyor needs to be appointed.

The fee is between £900 and £2,500. The same surveyor can work for both parties without bias to save money on two separate surveys.

Decorating

Many homeowners like to do their own decorating, while others prefer to leave it to a professional. Fees for a professional decorating job are likely to work out at about £150 per square metre. This typically includes all walls, ceilings and woodwork with two coats of paint on each surface. Wallpapering costs are about £14 per square metre.

Upgrading the Boiler or Electrics

A lot of homeowners decide to upgrade their boiler or electrics during a loft conversion. It’s the ideal time for either or even both, so it makes sense. The cost to upgrade to a combi boiler is between £2,500 and £5,000.

Updating your electrics depends on the extent of the work, but costs around £1,000-£3,500. A new consumer unit will cost you more than £500.

Some but not all properties may require extra work before or during a side dormer loft conversion.

Remedial Work to the Existing Roof

The joists on your existing loft’s floor are actually ceiling joists for the floor below. They are not designed to hold the weight of an additional storey. Therefore, they need to be reinforced with new structural-grade timbers.

Two to three structural steels or RSJs are fitted, and rafters may also be strengthened. This remedial work can cost around £1,200 to £3,000.

Chimney Alterations

Occasionally, a chimney may need to be altered as part of a loft conversion. The stack may need to be raised to ensure that it remains higher than the new ridge height.

Older chimneys may need new lead flashings , and there should be at least 40mm clearance between the chimney and any timber. Costs could be £800 to £2,000.

Water Tank Relocation

The cost to relocate a cold water tank in the loft is between £200 and £350. Homeowners can either retain the tank, move it to the eaves or upgrade to an unvented cylinder or combi boiler.

Strengthening of the Floor Structure

The loft floor is typically strengthened by attaching new floor joists to the original ceiling joists. These new timbers are structural grade C24, and dimensions are specified by the structural calculations.

Typical sizes are 47mm x 195mm and spaced at 400mm or 600mm centres. On average, a 3.6 metre length costs £16-£19 while a 4.8 metre length costs £21-£24.

Labour Costs and Timescales for a Side Dormer Loft Conversion

Cost and timescales are the two most pressing concerns for homeowners when building a side dormer loft conversion. Knowing what to expect can make the process less stressful.

Labour

Below are some of the daily rates for common trades that will be working on your side dormer, as well as some optional work.

• 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

In general, homeowners in London and the southeast will pay 15-25% more for labour.

Timescales

One of the biggest factors affecting timescales is the size and complexity of the dormer build. A simple side dormer on the hipped end of a traditional cut roof is usually very straightforward to build.

However, a lot of post-1965 roofs were built using pre-fabricated trusses. These triangular frames that support the roof run all the way through the loft. A lot of structural work needs to be done before the loft can be converted in these cases, adding to the timeframes.

Adding an ensuite to the build also increases the overall timescale.

Delays can also be caused by:

• Bad weather
• Unforeseen structural problems
• Party wall issues
• Planning issues
• Material shortages
• Long lead times on windows or bespoke items
• Access or parking restrictions
• Changes to the design once work has started

While the timescales given to you by your contractor are done so in good faith, it’s important to recognise that a lot of things can delay or stall your project. Keep this in mind and potentially add 2 - 3 weeks to the timescale, just in case.

Awkward access, a complex project or bad weather can slow down the installation process, which might add between 1 and 3 days onto the completion time.

Cost Factors for a Side Dormer Loft Conversion

There are some cost factors you can control, such as the level of finish, and there are some, like the amount of structural support needed, that you can’t. Below are some of the major cost factors for a side dormer loft conversion to consider.

Size and Shape of the Roof

The size and shape of your roof can have implications for the type of loft conversion that you want. Restrictions like minimum head height of 2.2 metres, measured from the top of the existing ceiling joists to the underside of the ridge beam, must be adhered to.

If the headroom is less than 2.2m, a solution is to drop the ceilings of the rooms below, increasing the head height in the loft. This extra work will add £20-£65 per square metre to the budget.

Current Condition of the Loft

The condition of the loft, including head height, existing obstructions, roof structure type and floor joist strength, can all impact the cost of a conversion, if major work is needed.

How Much Structural Support Is Needed

The degree of structural support required is determined by the structural engineer’s calculations. Each job is different as every property has its unique issues, strengths and weaknesses, but each must be able to withstand the increased load safely.

Whether Steel Beams Are Required

All loft conversions need steel beams; the only question is how many and the necessary dimensions to support the loft conversion.

A loft was intended to provide some ventilation to the house and evolved into a storage area; its construction was never designed to hold a whole extra storey. Therefore, significant structural support is needed for its conversion into habitable space.

Size of the Dormer

The size of the dormer will add to the cost of a loft conversion. Bigger dormers need more material, extra labour and an increased budget to fit out.

Standard of Insulation

The standard of insulation must meet Building Regulations, as this is one of the things that building control will inspect to ensure it complies. A minimum U-value of 0.15–0.18 W/m²K is required, typically achieved with 270mm of mineral wool or 150mm - 175mm of PIR board.

Quality of the Fixtures and Finishes

The quality of your finish and the fixtures in the loft will add to your costs. Whether you decide on a basic, mid-range or high-end look is determined by your preferred taste and budget.

Site Access

Limited or difficult access to the site adds to your project costs. If delivery vehicles, skip and scaffolder lorries can’t get close to the property, you’ll pay more in labour costs as things need to be moved manually. Or you end up with a smaller skip that needs to be changed more often. Parking restrictions also impact your budget if trades need permits while on-site.

Longer Scaffolding Hire

Project overrun can mean that you need to extend the scaffolding hire. Unavoidable delays due to weather conditions, material shortages or long lead times may stretch the timescale of the work and increase the cost of the scaffolding hire.

Plumbing

Adding an ensuite to the side dormer conversion adds an extra 10 - 15% to your costs. The level of finish in the bathroom in terms of sanitaryware, tiles and flooring choice can further increase the price.

Regional Labour Rates

Labour rates vary across the country, and some regions are more affordable than others for home improvement projects. Homeowners in London and the southeast usually pay the highest rates for labour, often up to 25% more than other parts of the UK.

Older Homes

Older homes may need extra work to bring them up to a suitable standard to accept a loft conversion. Sometimes DIY jobs done by former owners can impact the structure in a way that needs extra support to allow the loft conversion to be done safely. This is why hiring a professional loft conversion specialist is so important for homeowners.

Building Regulations for a Side Dormer Loft Conversion

Building Regulations approval is needed for all loft conversions, and there are several Approved Documents or Parts of the regulations that apply to a side dormer conversion.

Structural Stability

Part A refers to the structural stability and safety of the build, with the steel beams and floor joists being the focus. Due to the significant structural alterations taking place, it’s essential to comply with the requirements of this document.

Fire Safety

Part B relates to fire safety, including the need for 30-minute fire doors on escape routes. Requirements for materials to control fire spread internally and externally, as well as the need for mains wired smoke alarms, are part of this document.

Stairs

Staircase design is included in Part K of the Building Regulations. The loft stairs must adhere to a certain pitch, step dimension and headroom.

Insulation

The thermal U-value of the insulation for a loft conversion is set in Part L of the Building Regulations and is checked as part of building control inspections to ensure it complies.

Ventilation

Part F refers to the requirement to provide adequate ventilation within the loft conversion. It covers the installation of trickle vents, extractor fans and openable windows. The purpose of this document is to ensure good air quality inside the property.

Sound Resistance

Approved Document E relates to sound insulation in the loft. This involves inserting insulation to a depth of at least 100mm, particularly between the floor joists, to reduce the noise of footfalls for the rooms below. It also concerns sound insulation between bedrooms and bathrooms.

Safe Escape Routes

In addition to covering materials that prevent fire spread, Part B addresses the means of escape in the event of fire. The staircase from the loft needs to be protected and lead to a safe route out of the building.

Doors should be 30-minute fire doors, and windows should be easily opened without the need for a key or complex system.

Planning Permission

Although not every property needs planning permission for a loft conversion, every single loft conversion needs building regulations approval.

Building Regulations are a series of official documents created by the government that outline requirements for safe and compliant construction under the Building Act of 1984. They are not the same as planning permission.

Most side dormers are built under permitted development rights, which entitle the homeowner to make substantial changes without needing permission from the local authority. However, some homes, especially in conservation or other protected areas, do need planning permission.

Types of Side Dormer Loft Conversion

Not all side dormers are the same; there are some design options to suit most property types.

Flat Roof Side Dormer

The most common type of side dormer is a flat roof version, although it’s not completely flat and has a slight slope for water runoff. These dormers provide good head height and increased floor space.

Costs vary from £40,000 to £55,000 depending on size, location and internal finish. The final appearance should match the materials on the rest of the house to help the dormer blend in.

Pitched-Roof Side Dormer

A pitched-roof side dormer may have two or three sloping sides, the latter being more of a hipped roof. This helps the dormer blend in more with the original house rather than looking like an addition and maintains the property’s traditional appearance.

The pitched roof provides extra headroom as well as increased floor space. Costs range from £60,000 to £90,000.

Gable Dormer

Gable dormers are most often seen as small individual pitched dormers on the front facade of a property. Often requiring planning permission, they add natural light and area-specific headroom where a full dormer is not possible.

Individual gable dormers cost on average £800 to £1,200 and provide a traditional appearance to a property.

Large Dormer

A large side dormer is typically created to maximise floor space and head height in the loft. Semi-detached or detached properties, which this type of dormer suits, are allowed to add an extra 50 cubic metres of habitable space.

The final appearance of the property is determined by the size of the dormer and how well the cladding and roof match the existing. Done well, it can significantly enhance the home. Costs for a large side dormer are between £45,000 and £80,000.

Alternative Types of Loft Conversion

If you’re considering alternative types of loft conversion, check out rooflight, rear dormer, mansard or hip-to-gable options.

Rooflight Loft Conversions

A rooflight conversion is the most affordable option. It involves no major alterations to the roofline, and structural alterations are minimal. As a result, disruption to daily life is low during the process.

The work can be done under permitted development rights, and average costs are between £30,000 and £40,000. A drawback of this type of loft conversion is that, unless the existing roof slope is quite steep, getting decent head height is a challenge and can limit the use of the space.

Rear Dormer Loft Conversions

Rear dormers are the most common type of loft conversion and provide enough head height and additional floor space for a bedroom and, in many cases, an ensuite.

The build can be done under permitted development rights, and planning permission is only needed if the property is in a conservation area or the cubic metreage is above the permitted allowance.

Average costs for a rear dormer are between £45,000 and £55,000. The level of disruption is high, particularly in the first few weeks, as the roof section is stripped and also when knocking through from the loft to fit the stairs. Expect noise and dust.

Hip-To-Gable Loft Conversions

Properties with a hipped roof give you the option of a hip-to-gable loft conversion, providing significant increases in headroom and living space.

Most can be built under permitted development, but the process is very disruptive, particularly in the initial stages, as the roof slope is removed. The cost of this type of conversion is £45,000 to £60,000.

Mansard Loft Conversions

A mansard loft conversion is potentially the most disruptive loft conversion as the whole roof profile is restructured. However, the payoff is a massive amount of additional living space, although it doesn’t suit every property type.

The cost of a mansard conversion is around £60,000 to £80,000 and even higher in London. Planning permission is almost always needed as this conversion falls outside the remit of permitted development.

Hiring Contractors Checklist for a Side Dormer Loft Conversion

If you’re new to hiring a building contractor or just not sure what questions to ask a loft conversion specialist, our checklist will help you out.

Check the Contractor’S Experience with Loft Conversions

If you’re not using a loft conversion specialist, you need to check that the contractor has experience with building a side dormer. Loft conversions are not the same as general building projects and have particular requirements, so make sure you use someone with experience.

Ask for Photos or Examples of Previous Projects

Confirm that the contractors you’re speaking to have experience by asking for photos of previous work. Even better, ask to visit completed lofts and speak to clients about their build. Professional builders should be happy to agree to this.

Confirm Insurance Cover

You should confirm insurance cover by asking for evidence of public liability, employers' liability, professional indemnity or contractor’s all-risk insurance.

Find Out What’S Included in the Quote

Make sure you’re clear about what’s included in the quote. If it’s not clearly stated, ask if the architect's drawings, structural calculations and building control coordination are included in the price.

Check What'S Excluded

By looking at several detailed quotes, you’ll quickly notice that not all contractors include all the same things. Make a note of what’s excluded and ask why and how much it costs to include.

Confirm Estimated Timescales

Timescales for loft conversions are estimates, as the first few weeks are mostly roof work, which is at the mercy of the weather. However, you can confirm with the contractor that if there is a delay, they will attempt to complete as soon as feasible.

Ask Who Will Manage Each Stage of the Build

Some contractors offer a project manager for the duration of the loft conversion. This person liaises with the site foreman or head carpenter in the initial stages and stays updated via all trades as the work progresses. If there is no project manager, ask who will do this management role.

Make Sure There’S a Clear Payment Schedule

A payment schedule should be standard for your loft conversion, with set amounts being paid as various stages of the work are completed. This offers clarity and a clear timescale for paying for each part of the job.

Removing a Side Dormer Loft Conversion

On occasion, part or all of a side dormer may need to be removed, replaced or fixed to make it safe.

Poor Workmanship

Bad workmanship can make the side dormer unsafe, particularly if the problem is with the structural integrity of the build. If the work can be rectified and there are no safety issues, the dormer may not need to be removed.

Structural Defects

Any structural defects need to be assessed by a structural engineer and their recommendations followed. They will determine if remedial work is feasible and cost-effective. The extent of the problem will often dictate whether repair or removal is best.

Water Ingress

Failing flashings or broken roof tiles can allow water into the dormer; these are often cheap and easy to repair without removing the whole structure. Depending on how long water has been getting in, other elements of the build may need to be repaired or replaced.

Roofing Issues

Frequent leaks despite repeated re-roofing attempts most often mean the dormer needs to be removed and rebuilt.

Major Refurbishment Work

The price of major refurbishment work may be less cost-effective than removing the dormer and rebuilding it. As with a new build, get several quotes for the work to gauge which option is best.

Remember, you need specialist expertise to safely dismantle a side dormer. You also need to budget for waste removal, repairs and reinstating windows, insulation and internal finishes. All of which adds significantly to the final cost.

FAQs