Despite a huge growth in the number of basements being excavated, homeowners are still nervous at the prospect of the build itself. Veteran basement builder Robin Knowles (whose firm, Knowles, is responsible for some of London’s most luxurious retrofitted basements) explains how to assess feasibility, what to expect during the build, and his best design tip – quality, not quantity…
If you’re scoping out a potential new home and want to build a basement, how do you know whether it’s going to be suitable?
Look at what’s in the area – are there people doing basements already? You could also get a structural engineer into see if it’s possible. There are a couple of factors that make excavating a basement more difficult. Houses built on made-up ground – filled-in bomb sites, where 6m-deep craters would often be filled up with bricks and rubble – can be hard to build on. The age of the property is also a factor: the Victorians built pretty much on the earth, with foundations that aren’t that robust, so it’s easier to underpin; modern-day construction uses lots of concrete, making it harder.
Assuming you have your planning permission granted and the design side sorted, what happens next?
There are a lot of hoops you have to jump through, just to get on site. Getting party wall agreements can be tedious. Neighbours can’t say no, but they do have to give specific permission for putting foundations with steel down. If they don’t give permission, you have to put concrete underneath the foundations, then build a wall in front of that – which loses a bit of space internally.
What’s the basic procedure for excavating a basement?
Normally, you’d start at the front, and underpin as you go. There are two types of structural works, temporary and permanent, and sometimes the temporary works are harder to do than the permanent ones. The temporary works are a way of holding the building up while you do the underpinning: it’s carefully designed, with steels going through the wall, maybe some concrete piles drilled in to the ground. Then you dig underneath, so the whole building is effectively held in the air.
The permanent works consist of: the installation of the steels; the underpinning; installing the slab; and installing the retaining walls. This often goes hand-in-hand with the temporary works, rather than one after the other.
Our work ends once we’ve applied the internal waterproof membrane, so we leave a waterproof shell, ready for the electrics, plastering etc.
Is there a critical point in the whole build phase?
It’s all pretty critical to be honest. But I would say that the digging out shouldn’t be done too hastily, because it’s the earth that keeps everything stable. If you dig it all out [at once], you might look like you’re making progress – but I’ve seen horror stories where it has put the building at risk.
Are there lots of difficulties surrounding access?
It is a big issue, but most of it’s done with conveyor belts running all the way through, excavating the earth and then pumping the concrete in. In some cases it might have to be done manually, or you have to crane the machinery over the house to get to the back.
You quoted a rough cost of around £300 per square foot for building the shell, but what are some of the factors for the budget going up from there?
The price is dependent on the soil, and also the structural designs, which can vary greatly. It might be over-engineered, so you’ll end up spending a fortune on something you don’t need to, like special foundations. A concrete floor slab might be a foot thick, or two-feet thick; you might have a big steel when you don’t need a big steel. Inexperienced engineers tend to be very cautious in their design, which has cost implications.
I often have an input on the structural design, because I don’t like seeing money spent unnecessarily, and that’s where the client can really benefit. But not every basement contractor does it – lots just accept the design and get on with it – but I do it, because we have such a lot of experience.
Do you have a favourite project?
Good design can make a hell of a difference. I always like something where the floor space is opened up in a double basement. They can feel quite monotonous if it’s just one floor over another, but of you cut a bit of the floor out and create a double-height space it looks grander. People can think too much about gaining square footage, but sometimes you gain so much more if you take some of the floor space away. It’s quality over quantity.