Property Trend Going Underground

9 Mar 2021

Published in: House Beautiful, March 2021

When there's no space to extend outwards, digging downwards might be a great option.

If you're looking to add more day-to-day living space rather than a bedroom, then extending underneath the house or garden can add chill-out zones, an extra living room, a kitchen/ diner or even a home cinema.

Basements may not be the easiest renovation to undertake, but they offer a whole range of benefits. Here are six key points to bear in mind if you decide to extend your home underground... 

Is the extra space worth the cost? 

Cost, construction time and finished result all depend on how much excavation work is needed. If a house already has a basement, then it may just be a case of lowering the floor to allow for sufficient head height. Where houses don't have an existing basement, space can be dug out below the building, but it's a huge job so the amount of work and disruption shouldn't be under-estimated.

'Creating a new basement beneath an existing property is highly specialist work often requiring temporary support for the structure above as well as good access to dig and remove the soil safely,' explains Emily Bonner, structural engineer from London based organisation, the Basement Information Centre. 'The size of basement and property are critical factors, as are the ground conditions under the property. It’s clearly much more challenging to remove rock than earth, for example.' 

Do you need a return on your investment? 

Looking to the future, it's also worth deciding if you're going to get a return on your investment when you sell the property. Architect Neil Dusheiko, whose renovated basement is pictured above, says: 'The cost of excavating a basement is huge and it can be very expensive to get insurance for the work.' he says.

'Your project would need to be minimum of 40 square metres to get any kind of decent return on your investment. So work out whether you're going to be in that house long enough to recoup the money spent, and how valuable the space is to your lifestyle.’

How will you use the space? 

'The perimeter walls will be doing most of the work so you can create a lovely open-plan space, similar to a loft,' says Neil Dusheiko. 'You get very little noise in a basement, which means they're calm and peaceful spaces, and if there are no restrictions in terms of height, they can have quite high ceilings.'

Deciding early on how you're going to use that space, and whether you're going to separate it into rooms or leave it open plan, is essential, says Emily from the Basement Information Centre. ‘Those plans affect the Building Regulations that apply, as well as the waterproofing system and crucially, and often overlooked, the type of ventilation required. It’s better to install a system with the highest performance you're likely to need, rather than upgrade later; for instance, if you change it from a store room to a bedroom.'

Making sure basements are watertight is a key consideration, and as Emily says, good ventilation is particularly important if you're including a high-humidity area, such as a shower or bathroom. 

What permissions are needed?

Building Regulations approval will always be needed and planning permission may also be required. You're very likely to need to issue a Party Wall Notice to your neighbours, and it’s key that the requirements for fire safety and other important issues are considered as part of the early designs.

Get all the permissions early, as that way, says Emily, 'you'll know if there are any limitations set by your local authority that will impact on design.'

And what if you did want to have your own home cinema? 

Then you wouldn't be alone! Home cinemas are one of the biggest interiors trends right now. But it's important to bear in mind that instating electrics in a basement isn't a straightforward job and has to involve a qualified electrician - which your builder should organise. 

An alternative option

If digging beneath your home isn't feasible, consider the garden. 'Excavating under a garden is a possibility.' says Emily. 'It can be an excellent way of providing more space for your home while still having a usable garden or terrace above.

It’s likely to be far less expensive to build a basement under a garden than below an existing property but you'll need to establish whether you need planning permission. And one of the first things to think about is how you're going to access it easily, or safely escape from it in the event of fire, as your options might be limited by the existing arrangement of the property.' 

Contribution by - Emily Halliwell, structural engineer at The Concrete Centre and The Basement Information Centre