A home extension is a significant project to undertake whether you're in the building trade or not. It's not something a layman can do without expert help and guidance.

It's probably the one job where the London builder is guaranteed to get a call. Even the keenest DIY enthusiast is going to balk at the complexity involved in building an extension.

We're building more of them too. As fewer of us can afford a new house, we are opting to extend and improve the ones we have.

While our population keeps growing and no more houses are being built, we are increasingly extending the ones we have. While space is finite, planning consent is often granted for a reasonable extension, which makes it the ideal project.

Space is at a premium, especially in cities like London builders are increasingly moving towards home renovations as the supply of new build project and large scale renovations dry up. They are an ideal stop-gap for the local building firm to keep them going through the recession.

While expensive, the home extension is seen as an investment rather than a cost. That makes it more justifiable as an outlay, either to the homeowner or the lender who provides the finance.

A home extension done well will add value which can be realised once the property is sold. So even though it can be expensive, you don't lose the money, just invest it in bricks and mortar.

Extensions used to be a hard sell to homeowners. That was partly down to the expense, and partly due to the nature of the project. It's large and complicated and will inevitably result is disruption to the house. Many people didn't do them for that reason alone.

Now however, with finances tight, new mortgage approvals at an all-time low, many have no choice. Existing mortgage rates are anywhere from .75 percent upwards, where a new mortgage is more like 6 or 7.

That's if you can qualify for a new mortgage and have the 10 percent or more deposit that is now demanded on all new loans. That makes staying where you are quite attractive.

A large house can look much smaller once a child arrives. What was once perfect for your lifestyle suddenly feels cramped and just won't do. These kinds of situations are what London builders love. Unless the homeowner is in a good position, they won't be able to move so they will extend.

This can mean an addition to the house, a loft extension or interior remodelling. Loft conversions are popular, but are sometimes difficult to get through planning, especially if the street is in a conservation area or the council is a difficult one.

A rear extension is the favourite by far, as this is easier to get through planning. It can't be seen from the road, and as long as it doesn't inconvenience neighbours will be approved in the majority of cases.

These projects keep building firms in work until the economy recovers, which is why they love them. The layman can't do it, so if a family wants more space, they have to call in the builders. Simple as that.

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