Ask for planning and Building Regulations records
The documents expected depend on the age, size and location of the work. A lack of one document does not automatically prove the extension is defective, but it needs explanation.
Look closely at the junction with the old house
Cracks, damp staining and uneven floors around the join can reveal movement or poor detailing. Roof abutments and drainage around the extension are also worth inspecting.
Check what is hidden by finishes
A newly decorated room says little about foundations, steels or insulation. Where records are incomplete, a surveyor may recommend further investigation.