Planning Permission vs Permitted Development: Which Route for Your Extension?
Most Thames Valley extensions go through one of two routes: Permitted Development or full Planning Permission. Choosing the right route saves 6 to 12 weeks and 500 GBP in fees. This guide explains what each route means, the size limits for Permitted Development, when a Lawful Development Certificate is worth getting, and the cases where a full application is your only option.
Key takeaways
- •Permitted Development rights let you extend without full planning permission, within strict size limits set by the General Permitted Development Order.
- •Lawful Development Certificates are optional but strongly recommended. They confirm in writing that your build is lawful under Permitted Development.
- •Conservation areas, Article 4 Directions, and Listed Buildings disable most Permitted Development rights and force a full application.
- •A full planning application typically costs 258 GBP in fees and takes 8 to 12 weeks. A Lawful Development Certificate costs 129 GBP and takes 6 to 8 weeks.
- •Start with the Planning Portal checker or ask a local planning consultant or builder before committing to a route.
What is Permitted Development?
Permitted Development is a set of rights under the Town and Country Planning (General Permitted Development) (England) Order 2015 that lets householders carry out certain extensions and alterations without a full planning application. The rights are specific, limited, and depend on the type of property, its location, and the exact work proposed. For a typical semi-detached house in the Thames Valley outside a conservation area, you can add a single-storey rear extension up to 4 metres deep with a ridge height of 3 metres. Side extensions are allowed up to half the width of the original dwelling. Loft conversions up to 50 cubic metres for detached and semi-detached, 40 cubic metres for terraced.
Key Permitted Development size limits for houses
Single-storey rear extensions on semi-detached houses: up to 6 metres deep under the Larger Home Extension scheme, subject to neighbour consultation, or up to 3 metres deep without neighbour consultation. Detached houses get 8 metres under the Larger Home Extension or 4 metres without. Side extensions: maximum half the width of the original house, single-storey only, height matching existing eaves. Rooflights: must not project more than 150 mm above the roof slope. Dormers: permitted development for rear-facing dormers within volume limits, not permitted for front-facing dormers. Outbuildings: must not cover more than half the curtilage, maximum 4 metres ridge, 2.5 metres if within 2 metres of the boundary.
When you must apply for full Planning Permission
Full planning permission is required when Permitted Development does not apply. Common cases across the Thames Valley: conservation areas (Old Marlow, Windsor Castle Ward, Old Amersham, Eton High Street, and many village centres), Article 4 Directions (specific streets where local authorities have removed PD rights), Listed Buildings (always need Listed Building Consent plus often planning), flats and maisonettes (almost never have PD rights), houses converted from other uses (often have PD rights removed by condition), and any extension exceeding the PD size limits. Front extensions almost always need planning. Anything over two storeys needs planning.
What is a Lawful Development Certificate?
A Lawful Development Certificate (LDC) is an optional planning product that confirms in writing from the local planning authority that your proposed extension falls within Permitted Development rights. It is not a grant of planning permission. It is a formal confirmation that permission is not needed. The benefits are substantial. Conveyancers ask for LDCs when selling. Mortgage lenders sometimes require them. They protect you if the council later questions whether the work was lawful. They cost 129 GBP in England (April 2026 rate) and typically land in 6 to 8 weeks. For any PD extension, it is near-essential and pays for itself many times over at sale time.
How long each route takes
Full planning permission in a Thames Valley council: application fee 258 GBP for householder extensions in April 2026, statutory determination period 8 weeks, typical real timeline 10 to 14 weeks including validation and neighbour consultation. Lawful Development Certificate: fee 129 GBP, statutory period 8 weeks, typical real timeline 6 to 8 weeks. Prior Approval under the Larger Home Extension scheme: free, neighbour consultation period 42 days, plus determination, real timeline 6 to 8 weeks. Pure Permitted Development without any formality: zero wait, but you accept the risk of a later enforcement challenge without documentary protection.
Conservation areas and Article 4 Directions
Conservation areas across the Thames Valley are extensive. Windsor, Eton, Marlow, Old Amersham, Henley-on-Thames, Henley Bridge, Cookham, Sonning, and the centres of most villages carry conservation designations. Article 4 Directions go further by removing Permitted Development rights on specific streets or estates. Windsor has Article 4 coverage in the Castle Ward. Reading has targeted Article 4 Directions in student housing wards to control HMO conversion. Before you commit to a PD route, check the council's Article 4 register and the conservation area map on the local authority website. A single wrong assumption can cost you a retrospective planning application and refusal.
The decision framework
Start by checking whether your property has any constraints: conservation area, Article 4 Direction, Listed Building, or flat status. If any apply, full planning permission is likely your route. Next check the work scope against the PD size limits. If within limits, PD applies, and we recommend getting a Lawful Development Certificate. If outside limits, full planning is required. Marginal cases where the work is close to the limits benefit from a pre-application consultation with the local planning officer, which most councils offer for 150 to 300 GBP.
How Asuka247 handles the planning step
We review your property constraints before you commit to a scope. We draft plans that maximise PD rights where possible and switch to full planning only where necessary. We coordinate with local planning officers and submit applications for you. We handle prior approval and Lawful Development Certificates. If your work falls cleanly under PD, we can often break ground within 2 to 3 weeks of your instruction. If planning is needed, we plan the 10 to 14 week lead time into the overall project schedule.
Frequently asked questions
Do I need planning permission for a 3-metre rear extension?
On most detached and semi-detached houses outside conservation areas, a 3-metre single-storey rear extension is within Permitted Development. Get a Lawful Development Certificate to confirm it. Terraced houses, flats, and properties with constraints often need full planning even for this size.
Is a loft conversion Permitted Development?
A simple roof-light loft conversion without a dormer is often within Permitted Development. Dormer conversions are within PD only for rear-facing dormers within volume limits. Hip-to-gable and mansard conversions almost always need full planning permission because they change the roof shape.
What happens if I build without planning permission when I needed it?
The council can serve an enforcement notice requiring you to remove the work. You can apply for retrospective planning, but refusal is common and expensive. Four-year and 10-year rules provide some protection for long-standing work, but deliberately unauthorised work can trigger direct action.
Do I need planning permission in a conservation area?
Conservation areas remove most Permitted Development rights. Any extension, material change, or alteration visible from public view usually needs full planning permission plus conservation area consent. Individual trees in conservation areas are also protected.
How do I find out if my property has an Article 4 Direction?
Check your local planning authority website under the planning policy map or the Article 4 register. Thames Valley councils (RBWM, Slough, Reading, Wokingham, Bracknell Forest, Buckinghamshire, South Oxfordshire, West Berkshire, Spelthorne, Runnymede) all publish their Article 4 coverage online.
Can Asuka247 submit planning applications for me?
Yes. We work with local planning consultants and can submit Lawful Development Certificates, Prior Approval applications, and full planning applications on your behalf. We coordinate drawings, supporting documents, and any amendments the planning officer requests.
Need help on your block?
Asuka247 delivers Fire Safety Act 2021 remediation across the Thames Valley. Call 07931 118 247 or book a site survey.
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Published 2026-04-17. Last updated 2026-04-17.