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London Basement Extensions

Can I add a basement room to my London home?

Staircase leading into a basement of a Hampstead house extensions

Not every property is suitable for a basement extension. A house extension or a loft conversion usually presents a more affordable solution when it comes to extending your home. However, it is not always possible to extend above the ground. You might have limited garden space, and an existing loft conversion might already be in place. In this case, you might start thinking about expanding your home bellow the ground.

What to consider

Vital factors to consider before building a basement are the location, value, and the type of your property. An average basement extension in London costs around £5000 per square metre plus VAT. A large part of this figure is down to the groundwork: excavation and underpinning the foundations of the existing house. If your ground-level floor is made of solid concrete, it will need to be dug up, and a structural layer will have to be constructed to replace it. While a deck built of floorboards and timber joists is much easier to work with, it might need extra support from new steelwork resting on the existing walls. In prime North West London locations such as Hampstead, house prices are currently at least £14,000 per square metre. The cost of a basement might seem high, but if you live near the centre of London, it's still a lot cheaper than moving into a larger house in the area.

If you live in a terraced or a semi-detached house, the Party Wall Act procedure requires you to serve a notice to your neighbours. The foundations of adjoining dwellings might need structural work to keep them stable during and after the project. You should always discuss your plans with your neighbours at an early stage to reassure them that your basement will be built safely. Always hire an experienced construction company to carry out the work. All building companies should have liability insurance in place and will provide you with a structural warranty valid for up to 15 years from the date of completion.

Lightwell in a basement extension in Hampstead, London

Waterproofing
Underground water might cause pressure on the basement walls exploiting any weaknesses in the waterproofing. Even a tiny defect barely noticeable by the human eye can be enlarged by this pressure, which could ultimately lead to the basement being flooded. You can solve this problem by using a land drain to pump out any water collected around the basement perimeter.

Ventilation and Natural Light
Basement rooms need to be adequately ventilated, as set out in the Building Regulations. If there is no possibility of providing opening windows, the easiest way to circulate a supply of fresh air is to install a mechanical ventilation system. You could provide fresh air, as well as daylight, by lowering the garden and fitting glazed doors, with steps outside the door leading up to ground floor level. If this is not possible due to a lack of space, you could create a lightwell that will allow natural light into the basement through a partially glazed ceiling or a skylight extending into the garden. Alternatively, you could create a glass bridge connecting the basement with the ground floor.

Most common uses of a basement room
A basement room has the ability to absorb noise, which makes it the perfect room for teenagers. There they can play loud music without annoying the neighbours. Alternatively, you could use your newly gained basement space to house a home cinema, games room, or even a private gym.

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