Don't fall into a money pit. Discover the hidden places your money can go on building projects

There are some areas when you are either looking at a new property or thinking about renovations, extensions and refurbishments that always have to be approached with extreme caution. These are the 'black holes' of the building game, where your hard-earned cash can vanish quicker than a ship in the Bermuda Triangle. It is often very difficult to predict exactly how bad these black holes can be but the crucial thing is to be aware of their existence before you start work or exchange contracts.

Before you actually expose the foundations or start ripping off the wallpaper it can be hard to tell the extent of the problem, but a good builder should always flag them up for your attention and make you aware of a cost that may well go up. It is also worth getting specialist advice from the likes of structural engineers, damp and timber specialists or architects depending on the exact nature of your problem area.

Common 'black holes'

Damp
Damp is one of the easiest problems to predict. Your surveyor should pick up on the problem and a comprehensive survey will usually reveal the extent of the problem. Rising damp cannot go over 1.2 metres up the wall so if the damp course has been breached, the normal scenario is hacking off the plaster of the affected walls to 1.2 meters, injecting the walls with a damp-proof product and then re-plastering the walls with a Sika mix to ensure against any future penetration. It is expensive but straightforward. Complications can arise, however, if the damp is rising through your flooring. In which case, you may have to expose the foundations and re-pour a properly insulated concrete slab with a damp proof membrane lapped up into the walls.

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.

Rotten Timber
Another common area for extra costs is rotten or bug-infested timber, particularly in older properties. When you expose the joists you will quite often find some of them rotten, normally on the ground floor. This is not a major problem compared to timber infestation which could affect the entire structure of the property. Again you will not know until you hack off the plaster and lift up the floorings.

Plaster
The 'wet trade' of plastering is very skilled, very messy and can be quite expensive. Determining the nature of your plaster on your newly-bought Victorian home is a perilous business. You will only gauge its full condition once you have stripped off the many layers of tasteful wallpaper. In the best case scenario, holes can be bonded and filled, but in the worst-case scenario whole chunks of plaster accompany the paper from the walls. It is worth getting an idea of re-plastering costs even if it turns out not to the necessary.

Chimneys
Gas fires are very much in vogue these days but rightly require proper testing to ensure adequate ventilation. A common problem is that your chimney fails a smoke test so you have do drop a stainless steel flue down the chimney to ensure that no gas can escape into any upstairs flat or rooms. This will usually mean scaffolding is required but that may not be the end of problem. It is also common for the bricks of the chimney stack to collapse in on one another at some point in the chimney. This can mean opening up the chimney breast at various locations, some of which may be in your neighbour's flat above.

The party wall
One of the major stumbling blocks to a renovation project can be your neighbours. It is normally much worse if you are planning the works at the time that you are buying rather than planning changes to your existing property because your neighbours are more suspicious of you before you move in. The party wall act dictates that if you are doing any structural work which involves the party wall (the shared wall between you and your neighbour) you need to get a party wall agreement. This typically takes a minimum of two months. It can either be a written agreement between neighbours or involves a formal inspection by a party wall surveyor. The bad news is that your neighbours can insist on using a different surveyor than you do, which essentially means paying for the same work twice.

Get more advice on your building project in our Remodelling and DIY topics.

Felix Milns is a journalist, property developer and building contractor. His company, Zulufish Ltd, specialises in design-led refurbishment, extensions and conversions in west and south west London. For more information contact Felix@zulufish.co.uk.