A Beginner's Guide to Basement Conversions
Digging down might seem daunting (and can be expensive), but a basement can be a great addition to your property
Basement extensions are becoming more and more popular because when lofts or side extensions aren’t possible, people often still want to find more space. However, it is important to remember that excavation is expensive work to undertake – and the key is to understand the proposed use of the basement and the likely costs relative to property values locally.
Read a beginner’s guide to side extensions
Possibly the most important starting point is to be clear about exactly what you want a basement for. While this may seem obvious, the purpose of the basement will make a huge difference to the way that you design and build it and, accordingly, to the cost.
For example, if you are aiming to create an additional bedroom suite, the way that your basement is designed and built will be very different to that if you want a utility room, a home cinema – or a spot for band practice.
10 ways to find or add more space to your home
While the earth is naturally insulating, it is also generally damp and everyone’s dread about basements is that they should leak. The old way of creating a basement, known as ‘tanking’, was to try to form a completely waterproof box in the ground – like an inside-out swimming pool – but water pushing against a solid tanking barrier has a way of finding its way past or through in the end.
These days we tend towards a waterproof membrane inside the concrete shell with dimples to create space behind it for any water to be channelled to a central place (sump). By doing this, the waterproof layer is not under pressure and any groundwater that arrives can be safely drained or pumped away from the sump.
The basic costs of forming a basement – the digging out, the retaining structure and the damp-proofing system – mean that the starting point for costing a basement will be 50% to 75% higher than for basic work above ground. However, there are things that can be done to stop costs escalating.
Bringing direct daylight into a basement will certainly add cost – particularly if you want to add large areas of glazing or to dig out large exterior lightwells. So one way to keep costs under control is to use the basement for things that do not need daylight, such as a utility room. While a basement might seem like a very expensive way to create a utility room, if by doing this you can free up ground-floor space for a larger kitchen (for example) then it might well make sense.
However, unless you specifically want one of the basement functions that does not need daylight (sauna, music recording, home cinema, utility etc), most of the things people want basements for need daylight. And, in most cases, the key to a good basement design is all about how best to bring the daylight down and in, and how to avoid that claustrophobic feeling of being below ground.
Bringing daylight down to basement level by creating a void in the ground floor is probably the most cost-effective way of pouring daylight into a basement, as it avoids the cost of external excavation or windows.
In this example, light floods down from windows above ground level.
The potential downside, of course, is to give up some of your ground-floor space to become the void, and much will depend upon the relationship between the rooms above and below in terms of how great a sense of connection or separation you are after.
In this house, the lower-ground floor was previously very gloomy, but a combination of excavation outside, a void inside and lots of glazing means that daylight pours into this family space.
To make the loss of ground-floor space more useful, the void has been also utilised to install a beautiful staircase –creating an impressive feature with the added benefit of freeing up other floorspace.
The half-level excavation combined with the large areas of glazing have created a really impressive elevation for the back of this house. At night, the scene changes once more as the interior glows and the elegant staircase form is silhouetted.
If you don’t want to go to the expense of a large lightwell outside (see point 5) and your basement level isn’t entirely below ground, or if one part of it isn’t (as in this example, more of which you can see on the project’s page), it can be tempting to install high-level windows. These may sound like a good idea, but they can often have a ‘prison-cell’ effect and can make a basement feel claustrophobic.
Even though this room isn’t entirely below ground, a small external area opposite the doors into the garden has been excavated enough to incorporate windows to a standard sill level. It makes this part of the room feel more light and connected with the exterior – it’s an idea worth looking at if you’re trying to add light to an area of your extension that’s away from the main source of natural daylight, or to a basement room without floor-to-ceiling glass elsewhere.
Another slightly lower-cost alternative to an external lightwell is to form external steps leading up to ground level.
This basement living room seems lovely and light, and does not really feel like it is claustrophobic or underground even though the ground level is very nearly as high as the ceiling.
With this basement, much of the excavation has been exterior to the house to create a large external lightwell where the light can come down and into the basement.
The lightwell is large enough to provide space for outdoor seating, and full-height glazing to the interior basement rooms creates the impression that they are at ground level, opening out to a patio area. Clearly, however, the additional excavation makes this a more expensive option.
When you add a basement for additional living space, there is a danger that it can feel rather cut off. Careful design to make the most of the different levels can be a wonderful device, as can be seen with several of the previous images and also with this example.
Here, a lower-ground floor, which is not entirely below ground, has been created and extended to use as a kitchen/dining/living space. It opens out onto a terrace at the same level, with steps leading up to the garden.
On the floor above, the original living space opens onto the roof of the lower level, which acts as a balcony and has steps down to the garden.
What would you use a basement conversion for? Tell us your ideas or experiences in the Comments section.