Garden Room Building
Regulations London 2026

Floor Area Thresholds

The key threshold is floor area. Garden rooms under 15m² floor area are broadly exempt from Building Regulations when used as an outbuilding ancillary to a dwelling. Most single-person garden offices (3m × 3m = 9m²; 3m × 4m = 12m²) fall below this threshold. Between 15m² and 30m², exemption continues provided the structure is built substantially of non-combustible materials or is sited more than 1 metre from any boundary. Above 30m², Building Regulations approval is required.

These thresholds apply to non-habitable outbuildings. If the building is to be used as sleeping accommodation (a garden annex), Building Regulations apply regardless of floor area — because it becomes a habitable space subject to Part L, Part B (fire safety), Part M (accessibility), and Part G (sanitation) in full.

Part P: Always Applies

Part P of the Building Regulations (Electrical Safety) applies to all electrical installations in a garden room regardless of floor area or use. This means every electrical installation must either be carried out or inspected and certified by a Part P registered electrician, or submitted to Building Control for approval. The completion certificate produced by the electrician is the document you will need to provide when selling the property.