Basements and Big Digs: Building Below Ground

Where land is dear and the planners guard every roofline, the only way left to grow a house is often straight down. Here’s what a big dig actually involves, and what to ask before anyone breaks ground.

Land is the expensive part. In Gerrards Cross or Beaconsfield the plot under a house is often worth more than the bricks standing on it, and planners tend to guard ridge heights and garden space closely. So when a family needs another storey and can’t go up or out, the conversation turns downwards. A basement adds serious floor area without adding an inch to the footprint or the roofline. A gym, a cinema, a plant room, all invisible from the street. None of that means a basement will sail through planning. Most will need permission, several councils have written basement policies that limit how far a dig can spread or how deep it can go, and the view that counts is the local planning authority’s, so get it before anyone draws a section. But where the usual extensions are non-starters, below ground is often the only direction left.

What the dig actually involves

A basement is a civil engineering job wearing a house extension’s clothes. The ground doesn’t want a hole in it. It wants to close the hole back up, and everything about the job flows from that one fact.

Keeping the water out

Build below ground and you’re building below the level water sits at, for at least part of the year. The British Standard for below-ground waterproofing, BS 8102 if you want the number, describes three forms of defence, and they’re simpler than they sound:

• Barrier protection. A membrane wrapped around the structure, a tanking layer that works like a wetsuit for the building.
• Structurally integral protection. The concrete itself is designed to resist water, with dense mixes and sealed joints, so the structure is its own defence.
• Drained cavity. Accept that a little water may find a way in, catch it in a cavity behind the internal walls and floor, and lead it to a sump where a pump sends it away.

Common practice on a decent basement is to combine two of the three, belt and braces, because fixing waterproofing after the event means breaking out finished rooms. The drained-cavity route relies on pumps, and pumps are machinery: you’ll want a backup pump, battery power for cuts, a high-water alarm and a servicing plan, the same way you’d treat a boiler. There’s more on pumped drainage, covering storage, backup and what a power cut actually does, in our drainage guide.

Two people matter here more than any product. The structural engineer designs the hole, what holds it open and what holds the house up. The waterproofing designer is a specialist whose only job is keeping the finished rooms dry. The standard expects waterproofing to be designed for the site, not bought off a shelf. Ask who those two people are on your job. If nobody can name them, that’s your answer.

A basement is the one extension the neighbours will never see, though they’ll certainly know about the dig.

Your neighbours and the Party Wall Act

Excavating near somebody else’s foundations is exactly what the Party Wall etc. Act 1996 exists for, and the rule is fussier than the version people repeat over the fence. Distance on its own doesn’t trigger anything. Depth does. Section 6 of the Act sets two tests, and a dig only bites if it meets one of them:

• Within three metres of a neighbour’s building or structure, notice is due if any part of your excavation will go lower than the bottom of their foundations. Stay above that level and this test isn’t met, however close you are.
• Within six metres, notice is due if your excavation would cut a line drawn down at 45 degrees from the bottom of their foundations. Go deep enough and you meet that plane from further away, which is why a basement usually catches both tests at once.

Where notice is due, the Act asks for it at least a month before you start digging, with plans and sections showing how deep you are going. The neighbour can consent, or dissent and have surveyors appointed to settle the terms. Either way a written record of their property’s condition is normally made before work begins, which protects both sides if anything is argued about later. Which test applies on your job, and how the measurement is taken, is for your party wall surveyor to confirm. Other parts of the Act can also come into play if you are working on a shared wall itself. Appoint someone early, because those notice periods run to weeks and a dig can’t legally jump the queue.

What it does to the garden and the street

Honestly: for a few months, your garden is a working site. Hoarding along the boundary, a ramp or conveyor lifting spoil, lorries arriving to a timetable, wheel-washing to keep mud off the lane, sometimes a banksman walking vehicles in and out. Lawn and planting in the working area won’t survive it. A straight contractor says so up front and prices the reinstatement rather than mumbling about it. The reward is that when it’s over, the garden goes back and the street forgets. The space you’ve gained shows up nowhere but inside your own home. And because the soil, the groundwater and whatever’s buried in between decide most of the method and cost, the sensible starting point is understanding what happens below your home before anyone quotes for moving it.

A basement done well is quiet, dry and permanent. A basement done badly is the most expensive puddle you’ll ever own. The difference is settled almost entirely before the first bucket of soil comes up, in the engineering, the waterproofing design and the paperwork. So spend your patience on exactly the parts that feel least exciting.