Outbuildings and Garden Buildings: GPDO Class E PD Rules, Building Regs Thresholds and Electrical Supply Requirements

Summary

Garden buildings and outbuildings are one of the most frequently asked-about topics in planning and building regulations. The rules are often misunderstood because the PD thresholds and Building Regulations thresholds are different, and neither is particularly intuitive.

The most important distinction: permitted development covers whether the building can be built without planning permission; Building Regulations cover whether the structure needs to comply with safety and energy standards. A 25m² garden office is typically PD (no planning needed) but does need Building Regulations approval.

The electrical supply from the house to the outbuilding is always notifiable under Part P — this is a common oversight. Even a simple twin-and-earth run is a new circuit and must either be installed by a NICEIC/NAPIT-registered electrician or notified to Building Control.

Key Facts

• GPDO 2015 Class E (outbuildings) — Permitted development for outbuildings within the curtilage of a dwelling:
  • Maximum height: single-storey only; eaves ≤2.5m; overall height ≤4m (dual pitch) or ≤3m (flat/mono pitch)
  • Maximum size: no floor area limit for PD (just the height and conditions)
  • Must be ancillary to the dwelling (not a separate dwelling or business premises)
  • Not on land forward of the principal elevation
  • In conservation areas / AONB: outbuildings within 20m of house on side/rear are restricted; any outbuilding exceeding 10m² within this zone requires planning permission
• Building Regulations — Schedule 2 exemptions:
  • Completely exempt — Detached outbuilding ≤15m² floor area (if on or near boundary) — no Building Regs needed at all
  • Exempt if ≥1m from boundary — Detached building between 15m² and 30m² floor area, not used for sleeping accommodation
  • Building Regs required — Any building >30m² floor area; any building used as sleeping accommodation (regardless of size); any attached extension
• Sleeping accommodation — A garden room used for sleeping overnight is sleeping accommodation and requires Building Regs regardless of size. This affects: garden annexes, granny annexes, home offices with a bed.
• Granny annex — Even if PD and within 30m² exempt, a garden room used as a self-contained dwelling unit (with cooking, sleeping, bathroom) is likely to require separate planning consent (it becomes a separate dwelling) and Building Regs (habitable space).
• Electrical supply — Any supply from house consumer unit is a new circuit: Part P notifiable. Must include SWA (steel-wire armoured) cable buried at minimum 450mm depth or run through protective conduit in the garden.
• Sub-consumer unit in outbuilding — Garden buildings with an electrical supply should have a small sub-consumer unit (RCD-protected). RCBO protection is standard.
• Water supply extension — Extending water supply to outbuilding is notifiable under Water Supply Regulations (notify the water company). A stopcock is required inside the outbuilding.
• Insulation and Part L — A habitable garden room (home office, gym, studio) should be insulated. If Building Regs apply, must meet Part L targets (0.18 W/m²K walls for new build-equivalent).

Quick Reference Table

Detailed Guidance

Permitted Development Conditions (Class E)

The full conditions for outbuildings under Class E:

1. Position — Must be within the curtilage of the dwelling (i.e., the garden/land belonging to the house). Cannot be forward of the principal elevation (in front of the building line).
2. Height limits:
   • Overall maximum: 4m (dual pitched roof) or 3m (any other roof)
   • Eaves height maximum: 2.5m
   • If within 2m of boundary: maximum overall height 2.5m (this catches most garages and large sheds near the fence line)
3. Ancillary use — Must be used for a purpose incidental to the use of the dwelling (garden storage, home office, gym, etc.) — not as a self-contained residential unit.
4. Conservation areas and listed buildings — Additional restrictions apply. In conservation areas, outbuildings within 20m of a house (on side or rear) with floor area >10m² may require planning permission. Check with LPA.
5. Maximum curtilage coverage — Article 3 Schedule 1 Part 1 Class E condition: outbuildings cannot cover more than 50% of the land originally within the curtilage of the original dwelling (excluding the house footprint).

Building Regulations Thresholds (Schedule 2)

Completely exempt from Building Regs:

• Single-storey detached building: floor area ≤15m²; no sleeping accommodation

Exempt if ≥1m from boundary:

• Single-storey detached building: floor area 15–30m²; no sleeping accommodation; not used to store explosive or highly flammable materials; substantially of non-combustible construction if >1m from boundary

Building Regs required:

• Floor area >30m² — full Building Regulations compliance (Approved Documents A, B, C, F, K, L, P)
• Any sleeping accommodation (regardless of size) — Building Regs Parts B (fire), F (ventilation), and others as applicable
• Attached garages/outbuildings (these are extensions, not outbuildings)

Electrical Supply

All new electrical supplies to garden buildings from the domestic consumer unit are notifiable under Part P Building Regulations. Requirements:

Cable:

• Armoured cable (SWA — steel wire armour, BS EN 60502-1) is standard for buried or partially exposed runs
• 2.5mm² SWA single phase for most garden buildings (lighting + sockets up to 20A)
• 4mm² or 6mm² SWA if EV charger or large workshop supply needed
• Burial depth: minimum 450mm in areas liable to be disturbed; 600mm under roads/driveways
• Alternatively: run standard T&E in heavy-duty conduit at the same depths

Sub-consumer unit in outbuilding:

• Minimum: dual RCD unit with MCBs for each circuit
• Preferred: RCBO per circuit (individual circuit protection)
• Earthing: extend the house earth to the building AND provide independent earth electrode (copper earth rod) at the outbuilding for TT earthing system. Consult BS 7671 Section 706 for agricultural and horticultural premises (similar rules apply)
• RCD trip time: ≤40ms at IΔn for outdoor circuits

FENSA/CERTASS vs Part P:

• The electrical supply is Part P notifiable (Part P registered electrician or Building Notice)
• The outbuilding itself is not a "dwelling" for FENSA purposes
• Part P covers "electrical installations in dwellings" — strictly speaking, garden buildings are dwellings' curtilage; most inspectors treat the supply as Part P notifiable

Water Supply

If extending a water supply:

• Notify your water supplier (WaterSafe scheme) under Water Supply (Water Fittings) Regulations 1999
• Install a servicing valve (stopcock) inside the outbuilding
• Consider frost protection: pipe insulation, trace heating, or drain valve if property is unoccupied in winter

Planning Considerations for Granny Annexes

A garden building used as a self-contained dwelling (sleeping, cooking, bathroom facilities, separate from main house) is a "separate dwelling" in planning terms — not an outbuilding. This is not permitted development. Full planning consent is required.

However, a "granny annexe" that is ancillary to the main house (i.e., no separate postal address, connected to main house facilities, not let separately) may be permitted development if it meets the PD conditions. The practical test is use: is the occupant dependent on the main house, or self-sufficient?

Frequently Asked Questions

Can I use my garden building as a home office under PD?

Yes — a home office is ancillary use of the dwelling, which is PD under Class E. It does not require planning permission or Building Regulations (if ≤15m² or 15–30m² away from the boundary and not for sleeping). A practical, well-insulated garden office is not sleeping accommodation just because it has a sofa.

Do I need Building Regs for a 20m² garden room 2m from the fence?

No — a 20m² building that is ≥1m from the boundary is exempt from Building Regulations under Schedule 2 (as long as it is not sleeping accommodation). If it were <1m from the boundary, Building Regs would apply.

Does a garden building affect my council tax?

Potentially — if the garden building has its own utilities and amenities, the council tax valuer may consider it a separate "dwelling" and add a billing entry. In practice, this rarely happens for garden offices and studios. It is more common for fully self-contained annexes. If in doubt, contact your local council's valuation office.

Regulations & Standards

• GPDO 2015 Schedule 2, Part 1, Class E — Permitted development for outbuildings

• Building Regulations 2010, Schedule 2 — Exempt buildings classes

• Approved Document P — Electrical safety in dwellings (including electrical supplies to outbuildings)

• BS 7671:2018+A2:2022 — IET Wiring Regulations Section 706 (horticultural/agricultural similar guidance)

• Water Supply (Water Fittings) Regulations 1999 — Notification for extending water supplies

• Planning Portal — Outbuildings guidance

• HSE — CDM for small projects

• LABC — Building Regulations exemptions

• garden rooms — Garden room structural construction

• part p overview — Part P notifiable electrical work

• planning permission — Permitted development for extensions

• building regs exemptions — Schedule 2 exemptions in detail