Inside villages
and towns planning permission is likely to be granted for almost any plot, although there can be laws specific to a certain area. The total build area is normally around 70-80% of the plot size on 2 floors and is usually up to a maximum of around 400 sqm. This generally has to be 2.5 metres from plot boundaries.

Building in the countryside
Usually 4000 sqm is required for planning permission outside of village boundaries. The total sqm that you are allowed to build is set by individual planning authorities and follows a sliding scale. For example on the minimum plot size of 4000 sqm you would be allowed to build approximately 220 sqm, this then increases with the size of the plot. However this can sometimes be adjusted and is dependant on local rules and negotiation.
Generally a building has to be 15 metres away from the plot boundaries unless it fronts a municipal road. The exception to the 4000 m2 rule is if a plot of land fronts a municipal road.

General:
You must always have authorisation from the Forest Commission, if the plot is in the countryside and the lot must always have an access to an existing road. In order to get permission you must present detailed plans to the Commission to enable them to judge whether your buildingplans are in harmony with its surroundings and that it can and will be built according to all applicable rules of safety. To building lots near the sea different rules apply.