The standard spec is 1mt deep x 600mm wide however it is dependent on ground conditions. Your builder will dig what he feels is suitable from his experience and the building control officer will inspect and approve it if he is satisfied.If not he will request further excavation or if made up ground may request a raft type foundation.
Hope this helps.

If you are building the first floor on top of an existing building, min 1m deep existing foundation will be required by Building Control. This will be checked and approved by DS on site before commencing. If you are building a new extension, the soil type should be checked. You can find out from your neighbours who recently built an extension or dug a trial hole. If it is clay, the foundation will be min 1.2m deep from undisturbed ground level but it can be deeper if there are trees nearby. If the site is ballast, a min 1m foundation will be needed. There can be other reasons to make the foundation deeper such as drainage, filled ground, etc. Therefore the foundation should be designed by a professional and checked and approved on-site by LA.

All depends on the ground.. could be 1m deep x 600mm wide with or without steel.. could be piles.. if the ground is sand then it could be only 400mm deep.
Building inspector will let you know

A qualified designer will be able to determine the loading of the wall and give advice on the type of soil and impact of trees (present and removed - these can cause shrinkage and heave respectively).
In a shrinkable soil you would generally require between 750mm and 1m depth, dependent upon the modified plasticity index or volume change potential. In a non-shrinkable soil a depth of approx 450mm may be acceptable; this would be satisfactory in frost susceptible soils such as chalk. In all instances, the depth should provide a clean, firm and undisturbed surface.
Width will be dependant on the total load of the wall (KN/linear m) and in dense to firm soils can range between 250 (or the wall thickness if greater) and 850mm. In loose to soft soils with upto 30KN/m load, 400-600mm wide will be adequate. Above 30KN/m load, then specific calculations would need to be carried out. In very soft soils, specific design is required regardless of load. These are situations where Rafts, piles and ground pre-stabilisation can be considered, but designs for these type of foundations should not be undertaken without a proper site investigation and core sample testing to ascertain bearing capacity.
There are a number of simple field tests which can be carried out to determine the soils type, but you would be better to leave this to an experienced person.

The type of foundation is determined from the load bearing capacity of the ground. If this is a new Developement on virgin ground a proper desktop investigation as to the history of the ground should be carried out with subsequent trial pits dug with excavated material examined by a qualified structural engineer.
If the ground is found to be weak or has a high water table shifting sands or made up ground a suitable solution can still be designed to cater for the loads from the proposed design.
The foundation is probably the most important aspect of constructing a building why bypass a professional structural engineer?