5 Great Basement Conversion Ideas to Create Space & Add Value

Thinking of extending your home into the basement? Where do you start? And how much does a basement conversion cost? The Resident shares 5 great ways to use the space created with a new basement and answers all your burning questions with Subterranean Spaces, specialists in basement construction and subterranean design

1 Build an annex: Create a self-contained flat with its own entrance and one or two bedrooms, maybe more, depending on the footprint of your existing property. Use it as staff accommodation or somewhere for grown-up kids to live while they save for their first home.

2 Create a cinema room: Cinemas or games rooms are becoming popular in basements, mostly because you can sound-proof basements and turn the volume right up without disturbing your neighbours.

3 Get fit in your own gym or swimming pool: With fitness becoming increasingly important to time-poor Londoners, a basement gym or swimming pool with hot tub or sauna saves you trekking to the gym.

4 Build a bunker: You can even create a panic room or shelter with air filtration and water filtration systems. In this case, the basement extension provides both a versatile living space and a safe space with additional structural protection.

5 Underground parking: Underground garages have become a valuable addition to London properties in the last few years. A double-tier lift garage allows you to park two cars in the same space as one, or you can create a more substantial underground garage.

everything you need to know
about basement conversions

Why should I consider a basement conversion or extension?
Basement conversions create a more natural extension to the home, unlike loft conversions or extensions, which can look a bit false and are limited in terms of space. A basement conversion looks more natural, like it’s part of the original structure. Also, you can have what you want in a basement – a soundproofed cinema or a swimming pool, which isn’t possible in a loft conversion.

How much does a basement conversion cost?
A basement conversion costs between £4,500 and £6,000 per square metre, but it depends on the finishing (whether you’re having underfloor heating, for example). It sounds expensive, but in terms of adding value to your property – if you consider that each storey of a house in London can be worth upwards of half a million pounds, certainly over £200,000 – then you’re upgrading your asset quite considerably.

How do you get natural light into a basement?
You can get natural light into a basement via a lightwell or stairwell at the front, via the fire escape at the back or, if you’ve got a semi-detached property, a lightwell to the side of the property. You can even fit a window and front door down there depending on the property.

Another possibility is to use light-mimicking technology called Big Sky, developed in Finland by Light Cognitive, which is great lighting for things like bathrooms, kitchens and hallways, or if you have a deep basement conversion and you’re getting the light at the front but it doesn’t reach all the way back to the kitchen.

What are the main considerations before undertaking a basement conversion?
Always make sure that all your insurances are in place and that your contractor’s are too. Subterranean Spaces has its own engineers, architects and professional servicemen in house, and all the guarantees are supplied at the end when it’s all signed off by building control. We can do the whole thing in one package for you so you have peace of mind.

Also, waterproofing, draining and tanking are very important. Once the basement has been constructed, make sure that whoever does the waterproofing and tanking is an approved contractor, otherwise that will be a defect on the guarantee. We install Delta Membrane waterproofing systems and flood chambers, which are recognised as the best in the UK.

Subterranean Spaces are specialists in basement construction and subterranean design and development. For further information call the London Office on 020 3086 9431 or see


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