Calculating your basement extension cost in London / South East

We have already written a post on extension costs in London here, but basement extensions are a very different proposition. They can be a fantastic way to gain more square feet – when space is at a premium, which is why they are so popular in the capital. For this reason we decided to focus on London prices, as construction costs vary so greatly across the country. So if you’re reading this up in Newcastle, you won’t have to dig so deep (no pun intended!)

We’ll try and keep it as simple as possible to give you a very basic guide to calculating your basement extension cost. 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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Extension above by Tessa, architect from Hackney, London. Click here to see more and shortlist her for your home project.


How much does a basement extension cost?

We asked our community of architects in London for a figure for the build cost based on their experience.

*Figures have been updated in 2022 following recent price increases in constructions costs of approximately 30%, according to a survey of architects and designers on Design for Me.

Most varied between £3,900 – £5,200 per m2 (for mid-range finishes and not including VAT or professional fees). A big chunk of the cost is down to the groundwork: excavation and underpinning the foundations of the existing house, and damp-proofing, which is why it is more costly per square metre than a typical above-ground extension.



The figures above are based on mid-range finishes. Think painted walls, carpet or possibly engineered oak flooring. It also doesn’t allow for additional fittings such as bathrooms or kitchens. Skylights or large expanses of glazing would also increase the final cost.


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). £20,000 might be more realistic if you are looking for something long lasting or of higher quality.

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 would push the total cost up 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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