Basement extensions are a great way to add extra space to your home, often enough space that you don’t need to move house. You might want to add an extra living room, bedroom or guest annex, perhaps a games room or gym, even a pool. On the other hand a basement extension can be an incredibly complex construction project, especially thanks to the city’s unique underlying geology, and it obviously comes with serious party wall implications. Here’s some insight for you.
The challenges of construction projects on London Clay
When carrying out building surveying, London is a special case thanks to its geology. The city sits on a vast basin of sticky clay, the Bagshot Formation, once a warm tropical landscape and packed with amazing fossils. It’s amazing stuff, but it also has a high shrinkage potential and is widely associated with damage to buildings in the form of subsidence and heave.
Structural damage can happen when there’s a change in the clay’s water content, especially fast changes over a few weeks or months, or when the clay dries out in some areas under a building but not others. Changes in subsurface drainage like water leaks can cause issues, as can sudden changes like removing a tree that was previously keeping the clay relatively dry. All this can impact the feasibility and cost of a basement development.
Get your Party Wall Surveyor arranged quickly to minimise delays
Party Wall Surveying takes longer and is more complex than other surveying tasks, and several visits will be needed. You can expect to pay anything from £1500 upwards for this. Agreeing on a Party Wall Award for a basement extension is a long process usually involving input by the building owner’s team and the neighbours’ surveyors or engineers. It can include design changes and even extra investigations of the site. All this means the process can take anything from 4 weeks to 4 months from the time the Party Wall Surveyor is appointed. And that means you need to appoint them as early in the process as possible.
Party Wall Act – ‘Security for Expenses’ – £10,000 plus
The Security for Expenses part of the Party Wall Act says that your neighbour can serve a formal Notice on the building owner doing the works, asking for a sum of money called the ‘security’ to be held on account while the work takes place. This safeguards the neighbour if the work begins and is then delayed for a long time, and also pays out if damage happens to their property, so they can fund repairs. Security for Expenses isn’t cheap, starting from £10,000.
Subsequent use expense
In future, if your neighbour wants to extend their own basement, they’ll be able to use your newly built foundation or subterranean wall and won’t have to build their own, saving considerable amounts of money. The Party Wall Act says your neighbour has to pay for half the cost of your wall if they decide to carry out basement development.
The last thing you want is for properties either side of yours, or your own, to start moving. If there’s movement you need to know about it instantly. You need Movement Monitoring to detect and quantify movement and any damage caused by it. Most of the time monitoring kit is installed in the neighbouring owner’s place before the work begins, often fixed to the front, rear or side walls and set to deliver a weekly movement report. They’re so sensitive they can detect a millimetre of change.
Do you need special foundations?
Basements are unique. The Party Wall etc Act 1996 lays out two kinds of foundation for basement walls. There’s mass concrete, a thick column of concrete beneath the entire existing party wall and the corbel footing, or a more narrow column of concrete reinforced with metal. To build a special foundation you need consent from your neighbours, something you need to get early on in the process.
The right people to support you – Advising Engineers
Advising Engineers are experts called in by your neighbour’s Party Wall Surveyor. It’s their job to review the structural design, calculations, and specifications to make sure the neighbour’s property isn’t at risk. If the plans aren’t sound, they can be changed at this point to cut the risk. An advising engineer costs anything from £500 upwards.
Soil Investigation Reporting
Before the work starts you’ll need a soil investigation report. This will drill down into the land to the same depth as the planned basement to identify the exact make-up of the soil, which means you can plan for an adequate foundation. A desktop report starts at £500 and a site investigation costs upwards of £1000.
Corbel footings and underpinning
Your existing Corbel Footing will need to be cut back, and the rest of the footing supporting the property above will have to be underpinned, usually, a foundation underpin that forms the outer wall of the basement. This ensures minimal shrinkage at the base of the existing structure, which in turn means the load is safely spread.
The cost of extending a basement
The cost of all this depends on the type of property, its location and the size of the extension, but you’re unlikely to be looking at less than a quarter of a million pounds, often a great deal more. On the other hand the investment could be well worth it, usually costing less than selling up and moving. If you’re in need of building surveying in London, for a basement development or anything else, we’ll be pleased to provide all the expertise and professional support you need.