What does it involve?
You’re looking at spending up to £100,000 in London to deliver an extra floor under an average terraced house, depending on how much excavation is involved. Planning will take at least eight weeks, construction a further 16 weeks. You can create any kind of room you like: a kitchen/family room or a separate flat are the most popular options.
The area has to have head height of 2.3m to satisfy planning regulations. You’ll probably need to underpin foundations, damp-proof, and undertake a large excavation — definitely not a DIY job.
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Assembling your team
You will need a lawyer, a surveyor, a structural engineer, an architect, teams of underpinning specialists and damp-proof experts, builders and decorators. Ideally, either your surveyor or architect will act as project manager. Alternatively, specialist basement-conversion firms can be expensive but make the process easier.
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What will all these people do?
First you need to establish whether the project is viable. That means legal checks on whether you own the land under your property; if you live in a flat, you might have to buy the land from the freeholder. Then your surveyor should conduct a feasibility study, to establish what constraints on construction you have, such as sewers, electricity and gas supply. It may be necessary to bring in a labourer for a day to work with the surveyor and engineer to investigate the depth of your existing foundations, the level of the water table under your home and the soil type. They may also conduct a camera-guided tour of your sewers.
An engineer is critical; he will establish the minimum you need in terms of underpinning and other technicalities. As a rough guide, your bill at this stage will come to about £10,000: several hundred pounds for the lawyer, £2,000 for the feasibility study, £5,000 to £7,000 for the engineer’s work and just over £1,000 for the investigations. Make sure that you know what the entire project will cost, and accept that it is quite likely to run over budget.
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What next?
You will need to spend more money before building work begins. Your surveyor has to issue party-wall notices to your neighbours, as there is a strict process to follow to ensure you don’t damage their house.
Your neighbours will then instruct party-wall surveyors to safeguard their interests — billed to you. The architect will submit drawings to the council to obtain planning permission — they will cost about £1,000. Full construction drawings will cost about £5,000. The engineer and surveyor can do without these, though they are desirable if you want your space to look “designed”.
You also have to meet building- control standards, again regulated by your local authority. This planning stage is important to make sure you get exactly what you want, so work closely with your architect.
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Lighting
Lighting is going to make all the difference to your new space, and there are minimum legal light requirements that your professionals will be aware of. Windows opening onto a light well are traditional, but the wells require costly excavation. Structural glass incorporated into the basement ceiling costs up to £500 a square metre, and you will probably want etched or sandblasted glass for privacy. Glass walls and open stairwells are other options.
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The building work
Excavation comes first. Then the perimeter foundations will need strengthening; this will be overseen by the engineer.
You need a damp-proofing membrane to run under the basement floors and link into a membrane in the walls to create a watertight space. This specialist job (called tanking) will cost about £15,000.
Your project manager should make sure all workmen offer “all risk” insurance and provide guarantees, as these can be relevant when it comes to selling your property.
What else do I need to know?
Digging a new subterranean room is more expensive than turning a little- used part of the house into liveable space. If you decide to excavate, don’t stint on it — go for really good ceiling heights. Decorate in light colours, and consider a “statement” staircase, such as a spiral staircase.
The lowdown
Basement Information Centre (advice and contractor listings), 01276 33155, www.basements.org.uk
British Wood Preserving and Damp-proofing Association, 01332 225 100, www.bwpda.co.uk
London Basement Company, 020 8847 9449, www.tlbc.co.uk
Midland Tanking, 0116 233 8180, www.beautiful-basements.co.uk
Timberwise Basement Systems (Cheshire), 0870 225 7915, www.timberwise.co.uk
Trace Basement Systems (Glossop), 0800 389 9040, www.tracebasementsystems.co.uk
The Royal Institution of Chartered Surveyors, 0870 333 1600, www.rics.org