What is a Basement Impact Assessment?
A basement is a good way to get more space when you can't build out or up. But digging next to other houses carries risk, so many London councils ask for a Basement Impact Assessment (BIA) with the planning application.
What it looks at
The assessment checks whether the basement could cause problems for the neighbours or the area, and how those problems will be avoided. It normally covers:
- ground stability and movement, and the effect on the houses either side
- groundwater, and whether the basement will block or divert it
- surface water and flooding
How it's put together
It usually starts with a screening exercise to work out which of those issues need looking at in detail. Then there's a site investigation (boreholes, trial pits, sometimes monitoring the groundwater for a while), and finally a report setting out the risks and how they'll be managed.
The design and the BIA go together
The report has to match the structural design: how the walls will be held up during the dig, whether it's underpinning or piled walls, and how the basement will be waterproofed. It makes sense for the same team to do both, which is what we do.
The Party Wall Act will almost certainly apply as well, and a clear design and assessment make that process easier.
If you're considering a basement, talk to us early. It saves time at planning.
