Garden Room Sizes in London: Permission and Size Limits by Plot
The maximum garden room size without planning permission in London is governed by three linked limits under GPDO 2015, Schedule 2, Part 1, Class E: height (2.5m within 2m of any boundary, rising to 4m for a dual-pitched roof or 3m for any other roof beyond 2m from a boundary, with a 3m eaves-height cap throughout), position (not forward of the principal elevation facing a highway), and coverage (outbuildings plus extensions must not cover more than 50% of the land around the original dwellinghouse). There is no single floor-area cap in the GPDO itself — the limiting factor on most London plots is the 50% curtilage rule, not a fixed square-metre number.
Because London terrace gardens are typically 6–12m deep and 4–6m wide, the 50% rule and the height rules interact differently on almost every plot. As an MCIAT chartered architectural technology practice, we survey the garden, calculate existing coverage, and produce planning drawings that confirm exactly what size garden room a specific London garden can take before permission is triggered, across all 33 boroughs.
How big can a garden room be without permission?
Height is the binding limit, not floor area. Class E allows 2.5m within two metres of a boundary, 3m elsewhere with a flat roof, or 4m with a dual-pitched roof — and everything you build, extensions included, may cover no more than half the curtilage.
| Limit | Value | Class |
|---|---|---|
| Height within 2m of a boundary | 2.5m | Class E |
| Height, flat roof, elsewhere | 3m | Class E |
| Height, dual-pitched roof | 4m | Class E |
| Eaves height | 2.5m | Class E |
| Total ground covered by outbuildings and extensions | 50% of the curtilage | Class E |
| Forward of the principal elevation | not permitted | Class E |
| Building regulations threshold | 15 m² within 1m of a boundary, or 30 m² | Building Regulations 2010, Sch 2 Class VI |
Sources: the Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class E; the Building Regulations 2010 (as amended), Schedule 2, Class VI. Read at 1 September 2026. A garden room used as sleeping accommodation loses both the Class E position and the building regulations exemption, which is the single distinction that decides most London garden room questions.
Is there a maximum garden room size without planning permission?
Not as a single number. GPDO Class E does not set a maximum floor area for a garden room; it sets height limits, a no-build zone forward of the principal elevation, and a coverage limit. In practice the 50% curtilage rule is what stops most London garden rooms getting larger than roughly 20–30m², because gardens are shared with existing extensions, sheds, patios and hard standing that already count towards the 50% figure. On a generous suburban plot in Croydon or Ealing with a large, unbuilt-on garden, a much bigger garden room can stay within permitted development than on a small, already-extended terrace in Islington or Hackney.
What are the height limits for a permitted development garden room?
| Distance from boundary | Roof type | Maximum height |
|---|---|---|
| Within 2m of any boundary | Any roof type | 2.5m |
| More than 2m from all boundaries | Dual-pitched roof | 4m |
| More than 2m from all boundaries | Any other roof (flat, mono-pitch, etc.) | 3m |
| Any position | Any roof type | 3m eaves height (maximum) |
The 2.5m within-2m rule is the one that catches most London terrace gardens out. Because plots are typically only 4–6m wide, a garden room positioned centrally with 2m clear on both sides needs the garden to be at least 6m wide before the taller 3m/4m heights become available — otherwise it is capped at 2.5m regardless of roof shape.
Roof shape has a real effect on usable space once a garden room sits more than 2m from every boundary. A dual-pitched roof unlocks the full 4m height, giving more headroom for a vaulted ceiling or a mezzanine storage void, while a flat or mono-pitch roof is capped at 3m — often the practical choice on narrower plots where a pitched roof would otherwise overshadow a neighbouring garden. The 3m eaves-height limit applies regardless of roof shape or boundary distance, so even a dual-pitched design with generous ridge height still needs its wall-to-eaves line kept at or below 3m.
What is the 50% curtilage rule and how does it apply to garden rooms?
Under Class E, the total area covered by outbuildings and other permitted development additions (extensions, previous garden rooms, large sheds) must not exceed 50% of the “curtilage” — broadly, the private garden and grounds around the house, excluding the footprint of the original dwelling as it stood in 1948 or as first built if later. This is a cumulative test: if a house already has a single-storey rear extension and a garden shed covering 30% of the garden, only 20% of remaining coverage is available for a new garden room before permitted development rights are used up.
This rule is why we always survey the whole garden, not just the footprint of the proposed garden room, before confirming permitted development status. Two identical 20m² garden rooms can have completely different permission outcomes depending on what else already occupies the plot.
How is the 50% curtilage rule calculated on a typical terrace plot?
Take a typical Hackney Victorian terrace with a garden 5m wide and 10m deep — a curtilage of 50m² once the original house footprint is excluded. If the house already has a single-storey rear extension covering 12m² and a garden shed covering 3m², existing coverage is 15m², or 30% of the curtilage. That leaves 20% of the 50m² curtilage, or 10m², available for a new garden room before permitted development rights are used up. A garden room any larger than 10m² on this plot needs a full planning application, even though it would otherwise comply with the height and position rules.
This is why two homeowners on the same street can get different answers to "what size garden room can I build" — the calculation is plot-specific, not a fixed borough-wide number. It also means adding a garden room in stages, or after a previous extension, needs the same cumulative check every time: permitted development is a running total of everything built under Class E and related classes since the house was first built or as it stood in 1948, not a fresh allowance for each new project. We recalculate the full curtilage position from scratch on every survey, using the original construction date where the deeds or planning history confirm it, rather than assuming no prior development has taken place.
How does garden room size interact with building regulations thresholds?
Planning permission and Building Regulations are assessed independently, but for a London homeowner planning a garden room, the two size thresholds worth designing around together are the GPDO 50%/height limits (planning) and the Building Regulations Schedule 2, Class 6 floor-area bands (construction standards):
| Internal floor area | Planning (Class E) | Building Regulations (Class 6) |
|---|---|---|
| Under 15m² | Permitted development if height/coverage/position rules met | Exempt (no sleeping accommodation) |
| 15m²–30m² | Permitted development if height/coverage/position rules met | Exempt only with 1m boundary clearance or non-combustible construction |
| Over 30m² | Permitted development if height/coverage/position rules met | Building Regulations always apply |
The genuinely useful insight for London gardens is that a garden room can comfortably be permitted development on size and still trigger full Building Regulations at 30m² and above, or lose its Building Regulations exemption at 20m² if it can't achieve 1m boundary clearance on a narrow plot. Sizing a garden room well means checking both tables against the same survey, not just the planning rules in isolation. See our companion guide to garden room building regulations for the full breakdown of the 15m² and 30m² bands.
Worked example: a 4m x 5m garden room in Camden
A typical Camden Victorian terrace has a garden roughly 5m wide and 9m deep. A 4m x 5m garden room (20m² footprint) positioned at the rear, with 0.5m clearance to each side boundary, falls inside the 2m-of-boundary zone on both sides, capping height at 2.5m regardless of roof design. If the existing house already has a single-storey rear extension covering roughly 15m² of the original curtilage, the 50% coverage test needs checking against the full garden area before the new 20m² room is confirmed as permitted development. Camden also has extensive conservation area coverage; on designated land, outbuildings to the side of the house are not permitted development at all, so position relative to the house, not just the boundary, needs checking against Camden's conservation area map.
Worked example: a garden office in Wandsworth or Richmond upon Thames
Larger rear gardens in Wandsworth and Richmond upon Thames — often 8–12m deep — give more room to sit a garden office more than 2m from every boundary, unlocking the taller 3m (flat roof) or 4m (dual-pitched roof) height limits. A 4m x 6m garden office (24m²) sited centrally with 2.5m clearance on each side can reach 4m under a dual-pitched roof, subject to the 50% coverage test and the 3m eaves cap. At 24m² internal floor area, this room sits in the 15m²–30m² Building Regulations band, so 1m boundary clearance or non-combustible construction is needed to keep the Class 6 exemption — comfortably achievable given the 2.5m clearance already used to unlock the taller planning height.
What if the garden is in a conservation area or on designated land?
On designated land — conservation areas, Areas of Outstanding Natural Beauty, National Parks and World Heritage Sites, all of which are common across boroughs like Richmond upon Thames, Kensington and Chelsea and parts of Westminster — outbuildings positioned to the side of the house are removed from permitted development entirely, regardless of size or height. Rear garden rooms can still be permitted development on designated land if they meet the standard Class E tests, but the margin for error is smaller and councils scrutinise garden room applications on designated land more closely. An Article 4 Direction can go further, removing some or all of Class E rights borough-wide or street-by-street; always check the specific council's Article 4 map rather than assuming standard permitted development rights apply.
Croydon and Ealing, by contrast, have far less blanket conservation area coverage than inner boroughs like Camden or Islington, though individual streets and estates can still carry Article 4 Directions or sit within smaller, locally designated conservation areas. This is why a garden room size that is straightforward permitted development on one street can require full planning permission two streets away in the same borough — the underlying Class E rules are consistent nationally, but the designated-land overlay is applied street by street, not borough-wide. We check the specific address against the relevant council’s adopted conservation area maps and Article 4 register as a standard part of every garden room survey, rather than relying on a borough-level assumption.
Does a garden room size limit change if the house is a flat?
Yes — Class E permitted development rights apply to houses only, not to flats or maisonettes. If your property is a flat, any garden room in the shared or demised garden needs planning permission regardless of size, height or coverage, because the GPDO Class E exemption simply does not extend to flats. This affects a significant share of London properties, particularly converted Victorian and Edwardian terraces split into flats across Lambeth, Southwark and Hackney, where the garden may be attached to a ground-floor flat but the building itself is legally a flat, not a house.
How we size a garden room to stay within permitted development
Every garden room drawing package from Architectural Drawings London starts with a measured survey of the whole garden, not just the proposed footprint — capturing existing extensions, outbuildings and hard landscaping so the 50% curtilage calculation is accurate, plus boundary distances so height limits are confirmed against the real geometry of the plot. We check the property against the relevant council's conservation area and Article 4 maps before finalising a size, and cross-reference the resulting floor area against the Building Regulations Class 6 bands so clients know upfront whether a full Building Regulations application will be needed alongside the planning position. For the full set of planning rules behind these size limits, read our guide to garden room planning permission rules.
Frequently asked questions
What size garden room can I build without planning permission in London?
There is no fixed maximum size in square metres. The limits are height (2.5m within 2m of a boundary, up to 4m beyond 2m for a dual-pitched roof), position (not forward of the principal elevation), and coverage (outbuildings plus extensions must not exceed 50% of the garden). On most London terrace plots, the 50% coverage rule limits size before the height rules do.
What is the maximum height for a garden room near a boundary?
2.5m, if any part of the garden room is within 2m of a boundary. Beyond 2m from every boundary, the limit rises to 4m for a dual-pitched roof or 3m for any other roof shape, with a 3m eaves-height cap applying throughout.
How is the 50% garden coverage rule calculated?
It totals the footprint of all outbuildings and permitted development extensions already on the plot, plus the proposed garden room, and compares that total to the curtilage area around the original dwellinghouse as it stood in 1948 or when first built. If the total exceeds 50%, planning permission is required.
Can I build a bigger garden room in a bigger garden?
Generally yes, because a larger garden gives more clearance to reach the taller 3m/4m height limits (more than 2m from boundaries) and more headroom under the 50% coverage rule. Plot size, not a fixed square-metre cap, is usually the real constraint in London.
Do flats have the same garden room size limits as houses?
No. GPDO Class E permitted development rights apply only to houses. If your property is legally a flat or maisonette, any garden room needs planning permission regardless of size, height or garden coverage.
Does a garden room in a conservation area have different size rules?
The height and coverage rules are the same, but outbuildings positioned to the side of the house lose permitted development rights entirely on designated land, including most conservation areas. Rear garden rooms can still qualify if all other Class E conditions are met, but councils in conservation areas scrutinise applications more closely.
Is a 20m² garden room always permitted development?
Not necessarily — floor area alone doesn't determine planning status. A 20m² garden room can be permitted development on one plot and require planning permission on another, depending on existing garden coverage, boundary distances, roof height and whether the property is on designated land.
