How big can a garden room be without planning permission?
In England there is no floor area limit for a garden room built under Permitted Development. Its height, its position and how much of the garden is already built on set the limits, and the room still has to be a sensible size for a use that goes with the house. The 15sqm and 30sqm figures often quoted are Building Regulations limits, a separate question.
A garden room built under Permitted Development in England can be as large as the garden allows, as long as it is no more than 2.5m tall within 2m of a boundary, single storey, behind the house, and the room plus any other outbuildings and extensions covers no more than half the land around the original house. In National Parks, National Landscapes, the Broads and World Heritage Sites, outbuildings more than 20m from the house are limited to 10sqm in total.
Floor area matters for Building Regulations. A room of 15sqm or less doesn’t need approval. One of 15sqm to 30sqm doesn’t either, if it is at least 1m from every boundary or built substantially of non-combustible materials. Over 30sqm, it does. Neither limit is a planning limit, and a room can need Building Regulations approval without needing planning permission. The official guidance is linked at the end of this page.
In short
- Planning (England): no floor area limit. The limits are height, position and half the garden, and the room has to be for a use that goes with the house.
- Building Regulations: approval needed over 30sqm, or over 15sqm within 1m of a boundary unless built substantially of non-combustible materials.
- Many garden rooms are 5m × 3m or smaller; at that size they stay under 15sqm inside and need neither.
- National Parks and similar areas limit outbuildings more than 20m from the house to 10sqm.
- Scotland caps outbuildings at 8sqm in conservation areas and 4sqm at listed buildings. Wales applies the 10sqm rule in conservation areas too.
- A room that anyone will sleep in isn’t covered by Permitted Development and isn’t exempt from Building Regulations, whatever its size.
Two different limits
What limits the size of a garden room in England
| Planning (Permitted Development) | Building Regulations | |
|---|---|---|
| Floor area | No limit, except 10sqm more than 20m from the house in National Parks and similar areas | Exempt up to 15sqm; over 15sqm, up to 30sqm, exempt if 1m from boundaries or non-combustible; over 30sqm needs approval |
| Height | 2.5m within 2m of a boundary; 3m or 4m further away | No height limit, but single storey |
| Position | Behind the front of the house; not beside it in protected areas | Within 1m of a boundary matters over 15sqm |
| Coverage | Outbuildings and extensions no more than half the land around the original house | No limit |
| Measured | Externally | Internal floor area |
| Sleeping | Not covered; normally needs permission | Not exempt at any size |
Class E of the General Permitted Development Order 2015 and Schedule 2, Class VI of the Building Regulations 2010.
How big in practice
Common sizes and what they need (England, no one sleeping in it, 2.5m tall)
| External size | Floor area inside (approx.) | Planning | Building Regulations |
|---|---|---|---|
| 3m × 3m | 7.3sqm | Not needed | Not needed |
| 4m × 3m | 10.0sqm | Not needed | Not needed |
| 5m × 3m | 12.7sqm | Not needed | Not needed |
| 5m × 4m | 17.4sqm | Not needed | Not needed if 1m from every boundary or non-combustible |
| 6m × 4m | 21.1sqm | Not needed | Not needed if 1m from every boundary or non-combustible |
| 7m × 4.5m | 28.1sqm | Not needed | Not needed if 1m from every boundary or non-combustible |
| 8m × 4.5m | 32.3sqm | Not needed | Needed |
Internal area assumes walls 150mm thick; actual walls vary by company from about 100mm to 200mm. Planning assumes the room is behind the house, outside a protected area, and that the half-the-garden rule is met.
At 5m × 3m, a common size for an office with a sofa, a room stays under 15sqm and needs neither planning permission nor Building Regulations approval. The step that changes things is the next one up: at 5m × 4m the room passes 15sqm inside, so where it sits and what it’s built from start to matter.
The half-the-garden rule on a small plot
On a small plot, the half-the-garden rule can limit a large room. The calculation takes the land around the original house, front garden included, and allows outbuildings and extensions to cover up to half of it. On a terraced house with a rear extension already built, a large garden room can reach the limit. Permitted Development in England explains how it is measured.
Scotland, Wales and Northern Ireland
Scotland. Like England, there is no general floor area limit, but an outbuilding in a conservation area can have a footprint of no more than 8sqm, and in the grounds of a listed building 4sqm. Outbuildings can cover no more than half the rear garden, or half the front. For building standards, a room up to 8sqm is fully exempt if it is at least 1m from the house (or from every boundary) and has no bed, toilet, flue or wood burner, and one of 8sqm to 30sqm needs no building warrant but must still meet the standards. Scotland’s rules
Wales. No floor area limit in most places. In National Parks, National Landscapes, World Heritage Sites and, unlike England, conservation areas, outbuildings more than 20m from the house can cover no more than 10sqm. Building Regulations follow the same 15sqm and 30sqm limits as England. Wales’s rules
Northern Ireland. No floor area limit outside protected areas; the 20m/10sqm rule applies in National Parks, AONBs and World Heritage Sites. Building Regulations exempt rooms up to 15sqm at least 1m from the house, and up to 30sqm if it is at no point less than 1m from a house, a road or the boundary, or is built substantially of non-combustible materials. Northern Ireland’s rules
Official guidance
This page explains the rules in plain English. The official wording is in these sources, and where a council takes a different view of a particular case, the council decides, subject to appeal. An experienced garden room designer will walk you through the rules at the design stage, and planning consultants can advise where a site is less straightforward. But as the householder, it’s your responsibility to confirm your own position.
- Planning Portal: outbuildings and planning permission
- The plain-English guide to the rules in England, from the national planning service.
- Permitted development rights for householders: Technical Guidance
- How each rule is measured, from the Ministry of Housing, Communities and Local Government.
- Planning Portal: is Building Regulations approval needed for an outbuilding?
- The 15sqm and 30sqm limits explained.
- Building Regulations 2010, Schedule 2
- The exempt buildings, including small detached buildings (Class VI).
- mygov.scot: build a shed, garage or greenhouse
- The Scottish Government’s summary of the outbuildings rules.
- Welsh Government: planning permission for outbuildings
- The rules in Wales, set out as siting and height.
Common questions
- What size garden room can I build without planning permission?
- In England there is no floor area limit under Permitted Development. The room must be no more than 2.5m tall within 2m of a boundary, behind the house, single storey, and with other outbuildings and extensions cover no more than half the land around the original house.
- Is 30sqm the maximum size for a garden room without planning permission?
- No. 30sqm is a Building Regulations limit, not a planning one. A garden room over 30sqm needs Building Regulations approval, but it can still be built under Permitted Development if it meets the planning rules.
- What is the biggest shed or outbuilding you can build without planning permission?
- In England, there is no set floor area. Height, position and the half-the-garden rule decide it. In a National Park, National Landscape, the Broads or a World Heritage Site, outbuildings more than 20m from the house can cover no more than 10sqm in total.
- How big can a garden room be without Building Regulations?
- Up to 15sqm of internal floor area anywhere, or up to 30sqm if it is at least 1m from every boundary or built substantially of non-combustible materials, with no one sleeping in it.
- Does a bigger garden room need planning permission?
- Not because of its floor area in England. A bigger room is more likely to hit the half-the-garden rule on a small plot, and more likely to need Building Regulations approval.
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Updated October 2026 · Part of our garden room planning guide.
