FoundationsFoundations need to be dug according to a predetermined plan, and to have been accurately surveyed and set out. It is surprising just how often this is not done, and occasionally the ramifications can be very serious indeed, as completed houses turn out to be in the wrong place and subsequently have to be demolished. Your plans should identify all the load-bearing walls and the width of the trenches to be excavated. The depth of excavation is harder to predetermine and this is routinely decided by the building inspector on site. This is where things can get a little bit tricky, because if you have a difficult site, the foundation trenches may have to go down two metres, sometimes even more, below ground, which is expensive and potentially dangerous.
What your building inspector or warranty provider is looking for is principally a good bearing on solid ground. However, you can never be certain just what lies beneath the ground until it's opened up. This has led to professionals becoming more and more cautious about foundations and specifying loads more concrete or, increasingly, engineered or piled foundations.
The traditional method (ABOVE) involves pouring concrete to a depth of no less than 150mm and then building up to floor level with bricks and blocks.
Raft foundations (ABOVE) are thick, reinforced floor slabs, strong enough to build the house walls off.
The 'trenchfill' method (ABOVE) is the most common- it's quick and easy but uses more concrete than strip foundations.