Digging deep to add a basement

Much has been made of this year’s ‘race for space’, with home movers buying and selling to gain an extra bedroom, a bigger living area or a much-coveted home office. When you couple this with news that a detached house was the most popular property type bought by new home purchasers during the pandemic, it’s no surprise there is a big squeeze on space.

Many people buy a smaller property for its potential to extend but the traditional route is to go up – a two-storey extension or a loft conversion, for instance – but how about digging deep? It’s easy to assume that basement conversions are just for property high rollers, with A-list celebrities excavating to add a swimming pool, a home cinema or a gym complex, but can anyone add extra square footage by going underground? 

With land at a premium, basements are gaining in popularity as an extension option. If you are considering this route, a quick look at other houses like yours may give you a clue as to whether you’ll enjoy success – if the properties already have basements, your plans will start on a positive note. 

If you are not aware of a successful cellar conversion or basement excavation at neighbouring houses, you will need to work with a structural engineer and the local council on a feasibility study. They are likely to raise the following points:

• The Party Wall Act: neighbours will need to be notified about any planned work as part of this Act, as it may involve underpinning their party walls and inserting beams into shared brickwork.
• Permissions: your local planning officer and building control department will decide what’s possible and what will need planning permission.
• Evaluation: every aspect of your home and land, from soil type and the water table to access and service pipework will need appraising as part of the planning stage. If you don’t understand what heave and hydrostatic pressure is – and how they can affect basement plans – consult with the professionals.

Don’t forget that creating a basement isn’t a run-of-the-mill building project. Going underground requires a team of experts, including structural engineers, surveyors, architects, excavators and waterproofing professionals. This basement guide, created by SWJ Consulting, is a good read for those who’d like more detail about what adding a basement involves.

The need for specialist skills brings us to the cost of creating or converting a basement, and this will vary according to what you have already. If you’re converting an existing cellar, costs can range between £900 to £1,400 per square metre. If your basement is brand new and requires excavating, expect to pay in excess of £1,500 and £2,000 per square metre.

It’s wise to contact everyone who needs to be on board for an estimate before getting too carried away. Ask about their availability too – the best trades can be booked up for months and demand may also mean their prices rise (as will a shortage in building materials).

So will a basement end up paying for itself? The answer is: possibly. For the shrewd property owners out there, basements may present a way of creating an additional income stream. Xcavate Robotics is a company that works with homeowners to establish whether they can squeeze additional underground properties onto their land, with the build taking place under their garden. 

Its subterranean construction system is capable of adding a lower ground floor below an existing structure, and it can even carve out an entire one-bedroom apartment beneath a lawn – perfect as a property that can be rented out, or as an annexe for multi-generational families.

Even if you’re not looking to become a landlord, adding a basement can add as much as 20% to your home’s value, reports the HomeOwners Alliance. Already have a cellar? Even converting somewhere dark, dingy and demoted to a dumping ground will increase your property’s worth.

Ask us for advice and examples if you’re looking to buy a property with basement potential. If the idea sounds like too much hassle and you’d prefer the work to have already been completed, we can find you a home with more space than you have now.