Double Storey Extension Cost
Costs and advice on building a two storey house extension
A two storey extension is probably the best way of increasing living space and bedroom accommodation at the same time. If you find that your home is far too small then a double storey extension could be the right solution as long as you have the exterior space required. The cost of adding a double extension will vary hugely depending on exactly what you want. Obviously, the bigger and more complex the extension, the more expensive it will be. Your location plays an important role in the costs too. If you live anywhere near London, your extension will probably cost a lot more. A two-storey extension not only gives you more space, it is more cost-effective than moving house and cheaper per square metre than a single storey extension. If both storeys in an extension are the same size, then a double storey extension will cost around 50% more than a single storey extension (this is mostly due to the fact that a two storey extension spreads the cost of the foundations and the roof).
Example Costs for Two Storey Extensions
Things to consider when planning a double storey extension
While many house extensions can be built under permitted development rights, without the need for any planning permission application, there are a number of restrictions to bear in mind when designing the project. First of all, the new extension cannot be taller than the highest part of the existing roof and must no extend beyond the existing rear wall by more than 3m. The extension also has to be at least 7m from the property boundary and has to be constructed using materials which match the original structure. However, all extensions over one storey will need planning permission. So bear this in mind when considering a single storey extension versus a two storey build. You’ll also need to get Building Regulations approval for your two storey extension to show that the build meets certain minimum standards in terms of health and safety, energy-efficiency and sustainability. Each local authority has their own table of charges and the overall size of the extension will dictate the fees for building regulations approval.
Note that if any neighbours share a boundary wall with your property get them to consent formally to the extension, otherwise you will need a Party Wall settlement which will typically cost around £800 per neighbour. Failing to observe the party wall act means your neighbours can take civil action against you and stop further work, delaying your project and increasing your costs. Neighbours can also seek compensation and even require the removal of any work already carried out.
The rear of a property is usually the best position to add a two storey extension and will have the best chance of planning approval, though if there is a large outdoor area such as on a corner plot, there may be the potential for a side two-storey extension which will be looked upon favourably. A “one and a half” storey extension is another option to get around roof height restrictions, by extending into the roof space, or alternatively digging down into the ground and build a two-storey extension with a basement level without breaking height restrictions.
When designing the extension, remember that the style, size and building materials may be limited by your local authority planners. This is where a local and experienced architect will be well worth their professional fees as they can advise what you can and can’t do right from the start.
Doing it Yourself
A two storey extension is a huge project and will be beyond the scope for even the most experienced DIY enthusiasts. However, there is nothing stopping anyone doing the build themselves, as long as they can satisfy the planning department and the building regulations. At the very least an architect is recommended to assist with the design and planning stage, plus of course an electrician will be required for the addition wiring work.
The actual job begins with the labour intensive process of digging out the foundations and pouring the concrete. Then the external walls can be built, followed by the internal structures then the roof. A carpenter will be very handy at this stage to build the roof structure. Windows and doors would typically be installed next along with the first fix electrics and plumbing now the building is watertight. Finally, you can now break through from the existing house (this is the time to move out if you have not already done so), put steel supports in place and make the joins good. Then comes the plastering which should be left for at least a week to dry out before the decorating takes place. Next comes the second fix, electrics, plumbing and flooring, followed by kitchen/bathroom units installation as applicable.
This entire process will take a professional team around 14 weeks, if you are not an experienced project manager and are doing most of the work yourself, expect it to take much longer, particularly if you also have a day job to attend!
Adding a double storey extension bullet points
- Even if planning permission is approved, apply for a certificate of lawful development from your local authority in any case
- A two storey extension is only around 50% more expensive than a single storey, but offers 100% more space
- Two storey extensions often have more impact on neighbours so be very careful with the design and planning
- Getting planning permission for a two storey extension can be more difficult when compared to a single storey extension
Hiring a Tradesman Checklist
- Always get at least 2 quotes before hiring.
- Never pay the full amount upfront.
- Get the quote in writing.
- For any payment you make, always get a receipt.
- On more expensive jobs, ask for references.
- Check if the tradesman is a member of any trades associations.
- Make sure the tradesman has public liability insurance.
Will adding a two storey extension increase the value of my property?
Should I do a single or double storey extension?
Should I move out while extending?
Ground floor kitchen shower extension.
Submitted by Sam
Hi there, planning to build a new bedroom top of the garage.
Submitted by Wasim
Single story kitchen diner rear extension, the same width of the house. Currently, 1 small extension/sunroom that is not keyed into the house which needs knocking out which will be replaced by this extension and remove the wall from kitchen to sunroom. Wanting to be done asap and completed within a ...
Submitted by Daisy
We have had an L-shaped extension done onto our house. It has been roofed and glazed. The builder did not finish the walls up to roof height, and left the job as he was offered a contract in London, and said his mate would come around and finish the job. His mate came around and fitted barge-boards ...
Submitted by Rick
Conversion of a conservatory to a dining room with a bit more extension to sideways. Building plan in hand.