If you have a cellar where the ceiling height is at least two metres, you could easily turn it into a cosy basement room by renovating the interiors and adding lights and ventilation. You might still have enough space left to keep a few bottles of wine there, and by turning your new space into a lounge or sitting area, you will never have to carry the bottle up the stairs again.
If your property sits on a slope, you could take advantage of this by building a semi-basement, with part of the new area sat below the ground. Since you can add windows and sometimes even sliding glass door to half basements, it is much easier to bring in air and natural light. You might even be able to extend the space into the rear garden, creating even more extra room.
When building a full basement, the whole new area will be below the ground level. It involves excavation, underpinning, waterproofing, tanking and under-structure construction. Basements are good at absorbing noise, which makes them the perfect space for home cinemas, playing music, or practising the drum kit without disturbing the neighbours.
An average basement extension in London costs around £5000 per square metre. A large part of this figure is down to the groundwork: excavation and underpinning the foundations of the existing house. In prime North West London and North London locations such as Hampstead and Barnet, building a basement is still a lot cheaper than moving into a larger house.
If your property is 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. An architect will have to carry out a survey before preparing the structural and architectural plans.
Underground water will potentially cause pressure on the basement walls exploiting any weaknesses in the waterproofing, which could ultimately lead to the basement being flooded. This problem can be solved by running a land drain around the whole perimeter of the basement and pumping out any water collected there. Or, you can construct a cavity within the wall to trap the water and allow it to run down to a drain.
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. Another way to provide ventilation and daylight is to lower the garden and install sliding glass doors onto a small patio with the steps outside the basement leading up to ground floor level.