The regs for footings are fairly clear cut. There are specific widths and thicknesses depending on what you are building and the type of ground. There are also prescribed strengths of concrete such as C15, C20, C35 etc. The actual depth depends on the local subsoil. For our project we had to make the footings 600 mm wide and 300 mm deep with C20P (or ST4, GEN 3) concrete. The soil in our area is a rich loam down to about a metre where you hit a very solid sand and gravel subsoil. That would be the depth I needed to reach. My specifications for the foundations were approved at my "commencement of work" visit so it was just a case of marking out the lines of the trench and digging down to the subsoil.
June 2008: The 600 mm trench is marked out for the foundations.
The main issue with the groundwork was the location of the drains. The original drain ran along the side of the house exactly where the extension was planned so this would need to be re-routed around the new walls with connections to serve the various utilities. Moving the drains would be a major task and was a key point to discuss at that first meeting with the building inspector. My initial plan had been to move the drains first and then start the foundations, but the building inspector advised that I should get to ground level with the build and then run the drains alongside. This would maintain the integrity of the trench walls and be much more practical for pouring the foundations.
One of my key tools I bought for the project was the Dewalt road breaker. You can hire these for about £40 per day but if like me you have a lot of stuff to break up it is probably worth buying one. They hold their value too, so you might consider buying one for your project and then putting it on Ebay at the end. I bought a 9" angle grinder and some diamond stone cutting discs to cut the concrete. I also bought a mattock for loosening the the stony stuff near the top and and new spade. There was going to be a lot of digging on this project so I wanted to have the right kit at the start.
June 2008: The breaker made relatively light work of the concrete and the hard edges to the trenches made the dig fairly straightforward.