Gone are the days when basements were dark, dank places reached by rickety stairs. In the past 15 years, an increasing number of Londoners have been excavating underneath their properties to create exciting extensions.
© Tim Soar
"Extending downwards is a relatively new concept but a growing number of people who don't want to move are seeing it as a viable option," says Maggie Smith of The London Basement Company. "We're experiencing a 30 per cent increase year on year." The advantages of digging down are many.
First, a basement will have a limited visual impact on your property compared with a loft or extension. "In areas such as Kensington, where you have terraces of white stucco, you probably wouldn't get planning permission to build up but you would for extending down," says Tom Tangney, a partner at estate agent Knight Frank.
'Subterranean rooms are great if you want to preserve your garden and gain space'
A basic basement extension will increase the floor area of a two-storey property by half but, increasingly, people extend beyond the footprint of their homes into the front or back garden.
In some cases, multiple floors are submerged underneath the ground. "The average size of our extensions has increased from 400sq ft in 2004 to 700sq ft," says Smith.
The final advantage of basement rooms is their flexibility. Unlike loft conversions, they are accessible from the ground floor. "It's most commonly living spaces that people want but basements can be turned into anything from playrooms, kitchens and guest bedrooms to gyms, multi-media spaces and swimming pools," Smith adds.
© Tim Soar
Building downwards does involve extensive structural work and is the most expensive way to extend. "Work on the basis of £300 a square foot plus VAT for a finished basement," says Smith. So, for a 500sq ft room that would cost £150,000.
"In areas such as Fulham, Chelsea and Barnes, properties are selling for between £500 and £1,000 a square foot, so £300 a square foot for a new floor level is not bad. In these areas you can expect a 30 to 40 per cent immediate return but in less desirable locations you're likely to make your money back but not add value in the short term."
Don't cut corners
With all basement extensions it is vital that strengthening and stabilising of the foundations and waterproofing is done correctly. Employ builders with previous experience or a specialist basement company, which can either design and build your basement or create the structural shell, leaving the finishing to you or your architect. A basement specialist will give a free quote and feasibility study.
With some properties, including modern terraces and town houses, basement extensions are not viable. For others, the type of property will determine the amount of disruption.
"If the house was built before the Thirties, it is likely to have a timber suspended floor and we can dig under the house from the front, breaking into the floor above in the final stage," says Smith. "But with properties that have a solid floor we need to dig down from within, taking out a room on the ground floor."
Extending into the basement can take up to six months. But if you can stand the mess and cost, digging down is a great way to add space.
The dos and don'ts when you start digging deep
Creating a subterranean floor is the most expensive way to extend a property. While The London Basement Company suggests budgeting for £300 a square foot for a fully finished extension, Basement Force quotes £100,000 for the creation of a basement under the hall and reception room of a typical terrace house (both figures exclude VAT).
Savings can be made by employing a specialist basement company to carry out the main structural work and finishing the interiors yourself, although the excavating and structural work is the most expensive part of the job, costing between £175 to £225 a square foot.
© David Stewart
Also bear in mind that many people move out during construction, adding rental costs to the bill.
"In terms of financial gain, the basement is the last place to extend," says Tom Tangney, a partner at Knight Frank. "You will probably get back what you put in, but unless you live in a particularly high-demand area you won't make much of a profit."
Saleability also depends on the type of space you create and whether there is demand for larger houses in your area. "If you turn your basement into a multi-media room it might appeal to a high-net-worth individual with no kids but it is unlikely to attract a family," says Tangney. The key is to research your local market before you embark on the project.
Contact local estate agents to find out if similar projects in the area are seeing a return and create a room that will appeal to your market, or one that at least provides the flexibility of change of use.
Rules and regulations
Normally, you will need to get planning permission for a basement extension. In the past it has been easier to get this than for other types of extensions because of the limited visual impact but recently local authorities have started to tighten up their rules due to resentment from neighbours sick of the inconvenience of lengthy basement builds and concerns over structural damage to neighbouring properties.
The quality of the design is therefore essential and it is advisable to get expert help from either an experienced architect or a specialist basement company. Employing specialists will also ensure that your project complies with a number of building regulations, covering such things as ventilation, damp-proofing, electrics and water supply.
If you meet all these stipulations, then planning permission should not be a problem, although rules for properties in certain conservation areas and for listed properties are stricter.
If you live in a ground-floor flat, you can extend downwards as long as you own the freehold or have a signed letter from the person who does. If you are extending a ground-floor flat, a semi-detached house or a terrace property, the Party Wall Act 1996 will apply.
At least a month before work commences you must issue those living on either side or above you with a party-wall notice, giving them time to object to your plans. Present them with as much information as possible because if there is a dispute they can appoint a surveyor to protect their interests and you will have to pick up the bill.
“Lighting is crucial to the success of any basement extension,” says Tom Tangney of Knight Frank. “The more natural light you can get into the basement, the more enjoyable it will be to spend time in and the greater its appeal to future buyers.”
If you are extending below the existing footprint of your house, the only way to get natural light in is through light wells at the front or rear of the property, or by redesigning your garden so that it steps down to basement level at the rear.
Alternatively, reconfiguring the interior layout to include a double-height space will allow the basement to benefit from windows at ground-floor level. Once you extend beyond the existing footprint, the job becomes a lot easier as you can install rooflights or SunPipes that draw light from above.
If you install rooflights with pedestrian-loaded glass, they can double up as outdoor terraces, providing glowing platforms when the lights are on below. “I’ve even seen a glass-tanked fish pond double up as a rooflight,” says Tangney.
Artificial light is also important. “The lighting depends on the room function,” says designer Oliver Heath. “In kitchens, strip lighting can work well but if it’s a lounge then you can create cosy corners with side lamps.
Concealed lighting above or below cupboards will also give a nice wash of light. The effect is more natural because it’s like light that washes in through a window, as opposed to concentrated areas of light with dark patches in between.”
Things to avoid
* Don't scrimp on your waterproofing. If the damp proofing fails then it is expensive and disruptive to rectify the problem.
* Dig deep enough to ensure a decent head height, at least 8ft. If it is not big enough the space will feel dark, oppressive and claustrophobic.
* Stretch yourself to the limit. Once you are committed to digging down it makes sense to create the maximum amount of space you can afford. You can add extra space later but you will have to undo a lot of the previous work, which will be costly and disruptive.
* Don't overdevelop your house for the area that it is located in. If your street is mainly full of couples and single-child families you could make your house too expensive for the area. Also, look at the balance of rooms in the house.
Milk:studio Architects: (020 7229 7454; www.milkstudio.net); William Tozer Architecture & Design: (020 7734 6055; www.wtad.co.uk); Alastair Howe Architects: (01279 439640; www.alastairhowe.co.uk)
The London Basement Company: (020 8847 9449; www.tlbc.co.uk); Basement Force: (020 7924 1434; www.basementforce.co.uk); Cellarwise: (0808 145 7000; www.cellarwise.com)
Blustin Heath Design: (020 7739 6416; www.blustinheathdesign.com); Eco Age: (020 8995 7611; www.eco-age.com)
Planning portal: (www.planningportal.gov.uk); Party Wall Act: (0845 124 9685; www.partywallact.info)
KEEPING IT GREEN
Ground Source Heat Pump Association: (01908 665555; www.gshp.org.uk)