Permitted Development Calculator (UK)
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How to use this Permitted Development calculator
- Select your house type: detached houses get the largest allowances, terraced and end-terrace houses the smallest.
- Choose the type of work: single-storey rear extension, two-storey rear extension, loft conversion, or side extension. Each has different Permitted Development rules under the GPDO 2015 (as amended).
- Enter your dimensions: projection depth in metres for extensions, or estimated additional volume in cubic metres for loft conversions.
- Flag designated land: if your property is in a Conservation Area, AONB, National Park, or World Heritage Site, PD rights are more restricted.
- Read the result: a green flag means your project likely falls within standard PD limits; amber/red means you likely need full planning permission or the prior approval route.
What is Permitted Development in the UK?
Permitted Development (PD) rights let homeowners in England carry out certain types of home improvement, extensions, loft conversions, outbuildings, without needing to apply for full planning permission, provided the work stays within specific size, height, and location limits set out in the Town and Country Planning (General Permitted Development) (England) Order 2015 (as amended). PD rights can be removed by an Article 4 direction or a condition on your original planning permission, so always check your property's planning history first.
Rear extension limits
For a single-storey rear extension: detached houses can extend up to 4m without prior approval (up to 8m using the Neighbour Consultation Scheme); semi-detached and terraced houses up to 3m (up to 6m with prior approval). The extension must not exceed 4m in height, and eaves height is capped at 3m within 2m of a boundary.
For a two-storey rear extension: limited to a 3m projection, with the roof no higher than the existing ridge line and materials that match the original house.
Loft conversion volume limits
Loft conversions under PD are capped at 40 cubic metres of additional roof space for terraced houses, and 50 cubic metres for detached and semi-detached houses. Roof extensions must not be higher than the highest part of the existing roof, and materials should be similar in appearance to the existing house.
Designated land is different
Properties in a Conservation Area, Area of Outstanding Natural Beauty (AONB), National Park, or World Heritage Site face tighter restrictions: some rear extension allowances and cladding changes are removed entirely. If your property sits on designated land, treat any result above the base PD limit as "likely needs planning permission" and confirm with your local planning authority.
When in doubt, get a Lawful Development Certificate
This calculator gives a quick estimate, not a legal determination. If you plan to sell the property later, a Lawful Development Certificate (LDC) from your local council provides formal proof the work was lawful under PD: cheaper than a planning application and worth the modest fee for peace of mind. See our Construction Cost Estimator to budget the build itself once you know your PD status, or the Joist Span Calculator for structural floor sizing on loft conversions.
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Side extensions and outbuildings: the numbers that catch people out
A single-storey side extension must not exceed half the width of the original dwelling - measured at its widest point, not the plot boundary. On a semi-detached house with a 5 m flank wall, the extension footprint caps at 2.5 m projection from that wall. Two-storey side extensions are not permitted development at all on properties in Article 2(3) designated land. Outbuildings fall under Class E: total footprint of all outbuildings must stay below 50% of the curtilage (excluding the original house footprint). A garden office that takes a 40 m² plot past that threshold requires a full householder application - planning fee currently £258 in England as of April 2024.
The "original dwelling" definition changes your baseline
Permitted development limits measure from the original dwelling as built, not the current structure. If a previous owner added a 2 m rear extension before 2008, your PD allowance does not reset - that 2 m counts against your remaining limit. The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class A defines "original" as the condition on 1 July 1948 or at the date of first construction if built after that date. Buyers of 1970s estates frequently lose 1–3 m of apparent rear-extension space to undisclosed historic works.
What this calculator does not cover
This tool does not assess Building Regulations compliance - structural adequacy, thermal performance (Part L U-values: 0.18 W/m²K for walls, 0.13 W/m²K for roofs), or fire separation requirements. It also excludes Article 4 Directions, which individual councils apply to remove PD rights in conservation-adjacent streets without any national register. Before committing costs, run figures through the construction cost estimator and check party-wall exposure with the party wall notice cost calculator.
Frequently Asked Questions
Do I need planning permission for a loft conversion in the UK?
Not if it stays within Permitted Development limits: 40m³ additional volume for terraced houses, 50m³ for detached/semi-detached, with no roof extension higher than the existing ridge and materials matching the original house. Dormer windows facing the street usually still need permission. Always check for an Article 4 direction on your property first.
What is the Neighbour Consultation Scheme?
It is the "prior approval" route that lets single-storey rear extensions go beyond the standard 3m/4m limit up to 6m/8m. You notify the council, who consult your neighbours; if no valid objection is raised within 21 days, the extension can proceed without a full planning application.
Can permitted development rights be removed?
Yes, via an Article 4 direction (common in conservation areas) or a condition attached to your original planning permission when the house was built. Check your council's planning portal or request a copy of the original permission before assuming PD rights apply.
Does this calculator apply to Scotland, Wales or Northern Ireland?
No: this tool uses the England GPDO 2015 rules. Scotland, Wales and Northern Ireland have their own permitted development regimes with different limits; check the relevant national planning portal.
What happens if I build beyond Permitted Development limits without permission?
The council can issue an enforcement notice requiring you to modify or demolish the unauthorised work. After 4 years (10 years in some cases) enforcement action may no longer be possible, but this is risky and can complicate a future sale: always get a Lawful Development Certificate for anything borderline.