Checklist for a larger home extension prior approval
A single storey rear extension deeper than 3m on a semi, or 4m on a detached house, needs prior approval under Class A. Here are the limits, the information to supply and how the neighbour consultation timetable works.
A single storey rear extension deeper than 3m on a semi or terrace, or deeper than 4m on a detached house, needs the larger home extension prior approval route before you build. It is not a planning application and it is not a free pass. You tell the council what you intend, the council tells your neighbours, and if none of them objects within 21 days the work can go ahead. The checklist below follows Class A of Part 1 of Schedule 2 to the General Permitted Development Order 2015, which is where every limit in this process comes from.
Confirming the extension qualifies for the route
Six questions decide whether this route is open to you. Answer them before drawing anything.
Is the building a house rather than a flat or maisonette? Flats have no Class A rights. Is it unlisted? A listed building needs listed building consent whatever the extension depth. Is the site outside a conservation area, a National Park, a National Landscape, the Broads and a World Heritage Site? Larger home extensions are not permitted on that designated land. Have the Class A rights been removed by an Article 4 direction or by a condition on an earlier permission, which is common on newer estates? Is the extension single storey? And will the total area covered by buildings stay at or below half the curtilage around the original house?
Original means the house as it was built, or as it stood on 1 July 1948 if that is earlier. Extensions added by previous owners count against your allowance, so a house already extended 2m to the rear has less headroom than the plans suggest.
Depth, height and eaves limits to check
The limits are hard numbers with no room for rounding. Measure from the original rear wall, not from the wall of a previous extension.
| What is measured | Detached house | Semi-detached or terraced |
|---|---|---|
| Maximum depth from the original rear wall | 8m | 6m |
| Depth above which prior approval is needed | 4m | 3m |
| Maximum overall height | 4m | 4m |
| Maximum eaves height within 2m of a boundary | 3m | 3m |
Three further conditions catch people out. The extension cannot extend beyond a wall forming a side elevation of the house. The materials must be similar in appearance to the exterior of the existing house, which rules out a render finish on a brick house unless you are prepared to argue the point. And no verandas, balconies or raised platforms are permitted, which includes a raised terrace along the back of the new extension.
Measure the overall height from the natural ground level at the highest part of the extension's footprint. On a sloping garden, a flat roof that reads as 3.8m at the house can exceed 4m at the far corner.
Information the authority requires
The Order sets out exactly what you must give the council, and an incomplete submission does not start the clock. You need:
- A written description of the proposed development, stating how far it extends beyond the original rear wall, its maximum height and its eaves height
- A plan of the site showing the proposed development
- The addresses of the adjoining premises, meaning every property that shares a boundary with yours, including at the rear
- Your contact address, and your email address if you are content to use one
A fee applies, set by the fee regulations and uprated from time to time, so take the current figure from the published schedule rather than from an older guide. Most authorities take the submission through the national planning application service with its own form for prior approval.
Add a short covering note even though the Order does not require one. Set out the three measurements as a list, state the house type, confirm the original rear wall position and say that the site is not on designated land. It takes ten minutes and it prevents the most common cause of delay, which is a validation officer asking for a figure you did not write down.
The neighbour consultation scheme and its timetable
The council writes to the owner or occupier of each adjoining property, describing your proposal and giving them 21 days to object. You do not do this yourself, and you cannot shorten it.
Whether to speak to your neighbours first is a judgement. Most of the time it helps, because an objection from someone who has never seen the drawings is harder to answer than a concern raised early and designed around. A simple conversation and a copy of the plan before the council's letter arrives is usually enough, and the approach in this neighbour engagement template works just as well for a prior approval as for an application.
If no adjoining owner or occupier objects within the 21 days, the authority must tell you that prior approval is not required. At that point you can build, within the details you gave them.
What happens if a neighbour objects
An objection does not refuse the extension. It changes what the council has to do.
Where any adjoining owner or occupier objects, the authority must consider the effect of the proposed extension on the amenity of all adjoining premises, and then decide whether to grant or refuse prior approval. The assessment is limited to amenity, which in practice means loss of light, overshadowing, overbearing effect and overlooking from the new structure.
Matters outside amenity are not part of the test. Disruption during construction, the value of the neighbour's house, the boundary line, drainage connections and party wall issues are all dealt with by other regimes and are not grounds for refusal under this route.
If prior approval is refused, you have a right of appeal to the Planning Inspectorate, and you can also apply for full planning permission for the same extension, where the council assesses the scheme on its planning merits rather than on amenity alone. Neither route is quick, so the honest advice is to design for the neighbour relationship at the start.
The 42 day period and starting work
Two things can give you the green light. The council telling you that prior approval is not required or granting it, or 42 days passing from the date the authority received your complete information without any determination being issued.
- Submit the description, plan, adjoining addresses and contact details, with the fee.
- Wait for the authority to confirm the submission is complete, because the 42 days run from receipt of the required information.
- The 21 day neighbour period runs inside that window.
- If you hear nothing by day 42, you may proceed, provided the development matches the information you supplied.
- Build to the details given, and keep the council's letter with your house papers.
That last point matters more than it sounds. The permitted development depends on the works matching what you told the council, so a change of depth or roof height during the build takes the extension outside the approval. If the design changes, go back before the concrete goes in.
Where a tree with a Tree Preservation Order is near the works, prior approval does not touch it. Consent for works to the tree is a separate application, and a tree protection plan and method statement is what the council will expect to see.
An illustrative submission checklist
The following is illustrative, for a 5m deep flat-roofed extension on a 1930s semi.
- Confirm house type, that it is unlisted and that the site is not on designated land, with a map extract
- Confirm no Article 4 direction and no condition on an earlier permission removing Class A rights
- Establish the original rear wall from historic plans or the deeds, and record any previous extension depth
- State the three figures: 5m depth from the original rear wall, 3.2m overall height, 2.8m eaves within 2m of the boundary
- Calculate the proportion of curtilage covered by buildings after the works
- Plan showing the site, the house, the extension footprint and the boundaries, with a scale bar and north point
- List the adjoining premises by address, including the property behind
- Covering note repeating the three figures and the designated land position
- Diarise day 21 and day 42 from the authority's acknowledgement
If the extension turns out to need a full application instead, you can check which supporting documents your site is likely to need and draft them for your own review.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. A lawful development certificate is the only formal way to confirm that works are lawful. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
How deep can a larger home extension be?
Up to 8m from the original rear wall on a detached house and up to 6m on any other house, measured from the original rear wall rather than from an earlier extension. Prior approval is needed for anything beyond 4m detached or 3m otherwise. Overall height is capped at 4m, with eaves no higher than 3m within 2m of a boundary.
Can I use this route in a conservation area?
No. Larger home extensions are not permitted on designated land, which covers conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites. A full planning application is the route in those areas. Check as well for an Article 4 direction or a condition on an earlier permission removing Class A rights.
What happens if a neighbour objects?
The council must then consider the effect of the extension on the amenity of all adjoining premises and decide whether to grant or refuse prior approval. The assessment covers loss of light, overshadowing, overbearing effect and overlooking only. Construction disruption, property values, boundary disputes and party wall matters fall outside the test.
What is the 42 day rule?
Development must not begin until the authority tells you prior approval is not required or grants it, or until 42 days have passed from its receipt of the required information without a determination. If the 42 days expire with no decision, you may build, provided the works match exactly what you told the council.
Does prior approval cover a change of design during the build?
No. The permitted development depends on the works matching the description and plan you submitted. A change in depth, overall height or eaves height takes the extension outside the approval and exposes it to enforcement. If the design changes, go back to the authority before the work is built.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.