This section is done in two parts, the section that is made of cavity walls on top of the foundation and the actual groundfloor.
The first section which is a cavity wall is made of concrete blocks on top of the foundation and this wall is erected up to the ground floor level and DPC/Radon section. Do remember that the floor level has to be at least 225 mm above the outer ground. If you are going to use natural stone, bricks or any other exterior finishes, except for plaster you need to make sure that there is sufficient foundation for them and that they start at least 150 mm below the DPC level.
After the walls are done, you have to fill the inside structure of the inner walls. This is usually done by using good quality hardcore material like limestone.
Do not use any cheap material as the floor could subside during time and crack. After the structure has been filled, the hardcore material needs to be compacted by a vibrating plate to insure that there is no chance of movement in the future.
Next a layer of building sand has to be put on to the measure of at least 50 mm. This is done to make sure that there are no sharp edges showing from the fill material which could puncture the damp/radon barrier.
Depending on the type of ground floor you choose, you may not need any fill under it. For example, if you decide to go for a precast concrete or a beam and block floor or hollow concrete slabs, you do not need to make any fill for the underneath.
This obviously has be discussed with your architect/engineer to decide on the type of flooring you will choose for your individual project. Overall, a beam and block floor or Hollow concrete slabs will cost more than the normal poured concrete slab but do remember that if you have a big house and you are more than 900 mm above the ground before the floor is poured, it will be costing you a lot to fill the foundation. So, do your maths and see which one works out cheaper and better.
Do remember to place all your drains and pipes at this stage. Go to Drains/Pipes