Whatever the planning position - full permission or permitted development - every extension needs Building Regulations approval. Here's what that involves for a typical domestic extension, and where the structural engineering fits.
What the regulations cover on an extension
- Structure - foundations, walls, beams over new openings, roof; all needing engineer's calculations
- Fire safety - escape routes, fire resistance, detectors
- Thermal performance - insulation of walls, floor, roof and glazing
- Ventilation, drainage (including building near or over sewers), electrics and glazing safety
Full plans vs building notice
A full plans application gets the design checked and approved before you build - problems are found on paper. A building notice skips the plan check; the inspector judges compliance on site, and anything wrong gets rebuilt at your cost. For structural work, full plans is almost always the right call, and some Building Control bodies effectively require it where steelwork is involved.
The typical sequence
- Design drawings (yours or your architect's) → structural design and calculations (us)
- Building Regulations package submitted; plan-check queries answered (us)
- Approval issued; builder starts with approved details to work to
- Staged site inspections - foundations, drains, structure, completion
- Completion certificate issued - keep it; your buyer's solicitor will ask for it
Timber-frame extensions follow the same regulations with their own structural and fire detailing. Carports and small detached outbuildings have partial exemptions with strict conditions - worth checking before assuming either way.
Frequently asked questions
Does a small single-storey extension really need calculations?
If it has foundations, an opening into the house, or a roof - yes, parts of it need justification. In practice nearly every extension involves at least a beam into the existing house, and that beam needs calculations for approval.