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Garden rooms & outbuildings

Permitted development limits, building regs thresholds, power and insulation.

All guides
Stub — growing
Last reviewed
4 sources
Regulations
  • Outbuildings are permitted development if they don't exceed 50 % of the curtilage, are single-storey, eaves ≤ 2.5 m, ridge ≤ 4 m (dual-pitch) / 3 m (other), and the building is not in front of the principal elevation. [1]
  • Within 2 m of a boundary the maximum overall height drops to 2.5 m. [1]
  • Buildings over 15 m² with sleeping accommodation, or over 30 m² regardless, trigger building regulations. [2]
  • All fixed electrical installations must comply with BS 7671 and notifiable work needs Part P sign-off. [3][4]
Hints & tips
  • Before slab excavation or trenching the SWA supply — CAT & Genny sweep (HSE HSG47). The cable run from the consumer unit to the garden room almost always crosses incoming services. Full method in the Safety guide.
  • Slab base (non-habitable): 100 mm C25 concrete on 100 mm MOT Type 1, A142 mesh on a 1200-gauge DPM. Pad foundations are an option on stable ground.
  • Habitable garden room (year-round office, gym, studio): 150 mm C28/35 slab over 100 mm of rigid insulation on a continuous DPM, with A193 mesh. Building Regs Part L + Part C apply — talk to building control before you pour.
  • Run a dedicated SWA armoured cable from the consumer unit on a 32 A RCBO for a typical office; 6 mm² for runs under ~30 m.
  • Insulate to a U-value of 0.18–0.22 W/m²K in walls and 0.15 in the roof for a year-round usable room.
  • Site the room 1 m from any boundary if you can — easier rendering, gutter access, and dispute avoidance.

This app provides general UK guidance and material estimates only. It is not legal, planning, engineering or building-control advice. Always confirm requirements with your local planning authority, building control, utility providers, manufacturers or qualified professionals.