Generally the design for foundations of a single storey building is the same as that for a two storey extension. The important aspects include:
- depth to the bottom of the foundations below ground level,
- depth of concrete in the foundations,
- width of the foundation concrete,
- location of the wall within the width of the foundation and
- probably even more important, the nature of the ground under the foundation.
Unless the original foundations were laid within the last 5 years it will be necessary to expose the existing foundation in one or two locations so that the above aspects can be assessed by a structural engineer or the Building Control Surveyor before planning a first floor extension on an existing single storey part of the building.
Other important aspects to consider are:
- suitability of existing roof structure to act as a floor
- suitability of existing lintels over ground floor openings
- suitability of existing walls