A few of questions for you.
1. Will the garage be built onto the house, ie. an extension ?
2. If not, what construction method, trad. brick, concrete panel, conc. block & render ?
3. What is the approx size of the garage, over 30m2 will most likely require building regs.
4. Have you checked if you require planning permission ?
5. Will it have a flat or pitched roof ?
6. If using brick, will the walls be cavity - this effects the width of foundations.
With regards to foundations, the type and depth a dependant upon the ground conditions you have. Usually the foundations go down until the base of the excavated ground is resonably firm and able to bear the loads of structure to be built onto it. Too shallow and the structure may fail in the future. The width of the foundation is usually 3 times the width of the wall which will be built on it.
Usually, foundations go down up to 1m in depth for a traditional 'strip foundation' the bottom of which is filled with concrete and the walls are built off the top of this concrete pad. With a garage, the foundations are unlikely to be this deep, but gives you an idea of the sort of depths involved. See what the ground if like when excavation begins. Some ground is already firm and has good load-bearing capacity and therefore the foundations will not be as deep as ground which is 'made-up' and granular in its make-up.
Another option is to use 'raft' foundations, which is effectively a slab of concrete that covers the whole area of the proposed garage, with thickened edges where the walls will be constructed. This type of foundation can be used where the ground condition is poor as the raft spreads the load over a huge area.
Usually, the upper most section of the foundation is a few courses of brickwork below ground, where dense, frost resistent engineering bricks are used up until you reach Damp Proof Course, which is ideally 2-3 courses above ground level. Normal facing bricks can then be used above this level.
Sorry if this is patronising you in any way, but there is some info to dwell on.
Is this any use ?
