When deciding on the type of extension your architect has designed for you a lot of work would have gone into the design of the foundations to make sure the project is built on a stable footing. There are different types of foundations that can be used ie, a strip foundation is the normal type of foundation which is a trench which is excavated to the footprint of the new works and when approved by the local inspector is filled with wet concrete, and when hard is ready for the installation of the brickwork to form your new extension. Some times your works may be near large trees like oak or the ground is classed as unstable for a strip foundations, so the architect will design a pile foundation which consist of a pile drilling machine to drill the piles along the footprint of the works.
When the piles have been drilled the holes will have metal mesh inserted and concrete poured into the holes to form the piles. These piles are drilled much deeper than strip foundations to reach good ground. After the piles are completed trenches are excavated between the piles and a polystyrene liner is installed and a metal reinforcing frame is installed connecting the piles together, and more concrete is pour into these sections to form the foundations ready for the walls to be built. Normally when having a pile foundation the new sub floor will be of a block and beam floor vented below.
Another option when trees or drains are a problem is a raft foundation which means the the hole area of the build will be excavate to a shallower depth than the strip foundation, which would be to a depth of 1 meter or deeper depending on the quality of the ground. A raft could be from 300 mm to around 500 normally and when excavated again a metal reinforcing frame is wired together in layers with a ring beam installed around the edge of the works known as a toe, which would sit below the new walls. When the mesh is installed and wired together concrete is poured over all of the new mesh to form the raft. Rafts can be made into any shape to form concrete bases where normal foundations cant be used.
There are also different ways of getting over foundation problems caused by things like drains being in the way by installing concrete lintels over that area to protect the pipes, or by installing a steel beam and wrapping the steel in wire mesh and surrounding in concrete. If the design is carried out well you will overcome any problems that crop up.