Cost of an extension London

Cost of an extension London

Extension cost in London

London house extensions by Design for Me architects. Click on the images to see their profiles and shortlist them for your job.


Calculating your house extension cost in London / South East

We have already written a post already on house extension costs here, but we thought it would be helpful to have a separate one for an extension cost in London and the South East, as prices can be considerably higher (sorry).

I’ll try and keep it as simple as possible to give you a very basic estimate for your extension. However, we strongly recommend that you get professional advice from an architect and/or QS who can put together a budget estimate for your particular site and brief.

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Cost per square metre

From here at Design for Me, during Spring 2017, we asked our community of architects and design professionals; “How much does an extension cost in London”? Based on their experience, most answers were between £1,800-2,200 per m2. Please note that this doesn’t include VAT, professional fees or a kitchen (which we will go into in more detail below).


Extension costs per m2


Basic cost calculations for a single storey extension

Let’s assume you are taking advantage of the (currently extended) permitted development allowance and extending by six metres (read more on that here). A typical London Victorian terrace is about five metres wide, giving you a 30 m2 rear extension.

So if we take a cost allowance of £2,000 sq/m of 30m2 the basic build cost would be £60,000 

+ Then we need to add on 20% VAT = £12,000

Professional fees will be around 10-15%. So if we use 15% of £60,000 it will be = £9000


* So a six metre single storey extension in London / SE would cost approximately £80,000 (without the extras listed below) *



How much does a double storey extension in London cost?

For a double-storey extension, as a rule of thumb you can add 50% extra to the build cost of a single storey extension –


* So a six metre double storey extension in London / SE would cost approximately £120,000 (without the extras listed below) *



The figures above don’t account for expensive finishes and fittings. Instead think painted walls, carpet or perhaps engineered oak flooring and uncomplicated lighting and electrics. If you’re imagining tiling, fitted joinery and expanses of glazing, a kitchen or bathroom in there, you should expect prices to rise pretty quickly.


How much does a new kitchen cost?

We’d recommend budgeting upwards of £10,000 for your new kitchen (for £10,000 we are probably talking IKEA and low/mid-price range appliances).

How much do bi-fold doors cost?

We’ve included this as it’s such a popular addition to a house extension. You can read our article here for reviews and costs of the best sliding door companies, but expect upwards of £5,000 for around a 2.5m width opening.

Complex site constraints

Think about the additional difficulties (and costs) your builder would need to factor in. Does anything need demolishing to build your extension? How easy is it to access the rear of your property from the road to bring in tools and materials and take out waste? Will your new addition necessitate moving or adding drainage? Or other services such as a gas supply / moving the boiler etc? All of these things could push up the total cost significantly.


Controlling your budget

Getting professional advice is the best way to manage your budget. A good architect will work with you to make sure you are spending your budget in the right way and advise where costs would be unnecessary or could be minimised. For larger or more ambitious extensions, you may benefit from using a quantity surveyor too.

Make sure you get three to five quotes from recommended builders (your architect can help compile a tender list for you) to get a firm idea of the costs before starting work.


Finding the right architect

Choosing the right architect for your home project is a critical first step, but can also be a bit of a minefield. This is exactly why I set up Design for Me in the first place. All you need to do is tell us a bit about your extension project and we’ll help you find the right architect or architectural designer for free!


Emily  Design for Me

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