Bigger garden rooms: the 15 m² and 30 m² thresholds
Planning permission and Building Regulations are separate gates. Even a fully PD-compliant garden room engages Building Regulations sizing rules: over 15 m² internal the siting rules tighten (boundary distance / non-combustible cladding), and over 30 m² you are normally into full Building Regulations approval.
How this scenario checks out
A 6m × 3m flat-roof garden room, 2.0 m from the nearest boundary, run through the Permitted Development and Building Regulations thresholds our engine tracks:
✓ overall height ✓ eaves height ✓ distance to boundary ✕ other conditions ✓ Building Regs: floor area ✕ Building Control – how it actually works ! Part P – electrical notification ! Structural: roof loading ✕ Structural: roof loading ✕ Structural: roof loading
| ✓ overall height | Overall ≈ 2.86 m vs 3.0 m limit (flat roof, ≥2 m from boundary). |
| ✓ eaves height | Eaves ≈ 2.48 m vs 2.5 m max. |
| ✓ distance to boundary | ≥2 m from boundary (2.00 m) → 3 m flat-roof cap applies. |
| ✕ other conditions | Single storey, not forward of the principal elevation, and outbuildings must not cover >50% of the garden. Check your plot. |
| ✓ Building Regs: floor area | Internal floor area 14.2 m². Under 15 m² → generally not required (no sleeping accommodation). |
| ✕ Building Control – how it actually works | At this floor area Building Regs generally do not apply (no sleeping use); but Part P electrical notification still does (below), and if you later add sleeping space or plumbing the position changes. Keep this pack and your electrician’s certificate with the house papers. |
| ! Part P – electrical notification | In England & Wales the consumer unit + supply connection are notifiable under Part P and must be installed/certified by a registered electrician. This tool outputs a spec, not a DIY wiring guide. |
| ! Structural: roof loading | Roof overhang cantilevers the joists ~600 mm – size the joists/firrings for the cantilever moment as well as the span and maintenance load. |
| ✕ Structural: roof loading | Flat-roof joists must carry an imposed maintenance/access load (≈0.6 kN/m² or a 0.9 kN point load, BS EN 1991-1-1) – verify sizing against span tables / an engineer. |
| ✕ Structural: roof loading | Deck tip: the roof runs 40 mm over 3 full OSB boards. Trim the back overhang to 60 mm and the deck lays with zero rip cuts. |
Common questions
Does a garden office need Building Regulations approval?
Under 15 m² internal and without sleeping accommodation: normally exempt. 15-30 m²: exempt only if at least 1 m from the boundary OR built substantially of non-combustible materials. Over 30 m²: full approval in most cases. Sleeping accommodation always triggers Building Regulations.
Is the 15m² measured internally or externally?
Internal floor area. A 6 × 3 m external footprint is roughly 15.5 m² internal once insulated walls are deducted - right on the threshold, which is why our engine reports the exact internal area for your dimensions.
Do electrics need sign-off even if the building is exempt?
Yes - Part P applies regardless of the building's exemption. A registered electrician must design, connect, test and certify the wiring, notified to Building Control.