The basic rules for foundations relate only to walls that are placed centrally on foundations – which ensures that they will safely transmit loads AND are built on good quality ground soil – NOT heaving soil or shrinkable clay. So if there are ground issues on your site, or special foundations have to be designed by an engineer for some other reason, you cannot rely on the dimensions specified below.
Basic, uncomplicated foundations should be constructed with concrete that has a compressive strength of at least 10 MPa at 28 days, OR concrete that is mixed proportionately by volume in the ratio 1:4:5 cement:sand:stone. Mixing by volume involves carefully measuring out the materials in the same-sized container. A wheelbarrow may be used, but it is not a suitable method for large building projects.
Continuous strip foundations should be at least 200 mm thick,- unless laid on solid rock.
The width of continuous strip foundations should be at least 600 mm if the foundation is for load-bearing or free-standing masonry walls, or a timber-framed wall that supports a tiled or thatched roof (which should, of course, be constructed according to the building regulations), OR 400 mm if the wall is a non-load bearing internal wall or a timber-framed wall that supports a metal sheet, fibre-cement sheet or light metal-tiled roof.
If a strip foundation is laid at more than one level, it is important for the higher portion of the foundation to extend over the lower portion for a distance that is equal to at least to the thickness of the foundation. If there is a void between the top section and lower section, you will need to fill the void with concrete that is the same strength as the concrete used for the foundations.