Building black holes

Subsidence
When damage is caused by subsidence, determining the exact nature of the remedial work is harder to predict. Diagnosing the problem is easy: look for cracks in the fabric of the building (not just the plaster) which will have been caused by the building slightly moving in the past. Much of London, for instance, is built on a clay base so many older properties have experienced subsidence to some degree. A structural engineer will require your builder to dig 'trial pit pigs' to expose the foundations so that he can ascertain how much remedial work is required. Most remedial work happens underground but there is also the necessary re-pointing of the affected brickwork and some potential for replacement lintels, windows and door frames.

Foundations
As with subsidence problems, the building foundations cannot be properly checked until they are exposed by a 'trial pit dig'. If you are planning a rear or side extension, your structural engineer will specify the depth he would like your new foundations to drop to (typically 1.5 to 2 metres). However, if the foundations of the existing building are not deemed to be sufficient when they are exposed, you may well need to underpin them. By digging out under them and pouring new concrete pillars, you further strengthen and support the existing foundations.

Drains
Typically a problem when you are adding a rear or side extension, changes to the drainage of your building can get complicated and expensive. Normally the manhole cover and inspection chamber for drains will be located exactly where you want to put your new extension. Complications can arise when the mains drainage is going to be affected by the new depth of your foundations. This can involve long and costly work to re-structure the layout of the drains. Again, this is a problem that cannot be predicted until you have broken ground.

Roofing
A major concern when people are buying properties is the state of the roof. However, this is generally one of the easier problems to predict. A good builder, surveyor or roofing specialist should be able to advise you of the extent of the problem. One exception is when you are doing a loft conversion; you may be able to get away with not replacing the joists and slates on the front of the roof but sometimes on closer inspection these may need to be replaced. Ask your builder for a price in advance just in case.