Key Takeaways
- A single-storey rear extension up to 6 metres (attached) or 8 metres (detached) beyond the original rear wall, without full planning permission.
- This is a notification route, not an application for permission. You are telling the council you intend to use a permitted development right.
- The council notifies adjoining neighbours and gives them 21 days to object, then has 42 days in total to decide.
- If the 42 days pass with no decision, prior approval is deemed granted and you may build.
- The fee is £249 from 1 April 2026, against £548 for a householder planning application.
- Not available on Article 2(3) land, which includes every conservation area, nor on listed buildings, flats or maisonettes.
The larger home extension scheme is the cheapest way to add a deep single-storey rear extension in London, and the most commonly misunderstood. It is not a shortened planning application. It is a notification that you intend to exercise a permitted development right, and the council's power to say no is far narrower than most people assume.
What the scheme allows
It doubles the standard permitted development depth for a single-storey rear extension, from 3 metres to 6 metres for a terraced or semi-detached house, and from 4 metres to 8 metres for a detached house, measured from the original rear wall. "Original" means the house as it was on 1 July 1948, or as first built if later, so a previous owner's extension counts against your allowance even though you did not build it.
| Limit | Standard permitted development | Larger home extension (prior approval) |
|---|---|---|
| Maximum depth, detached house | 4 metres | 8 metres |
| Maximum depth, any other house | 3 metres | 6 metres |
| Maximum overall height | 4 metres | 4 metres |
| Maximum eaves height within 2 metres of a boundary | 3 metres | 3 metres |
| Storeys | Single storey | Single storey |
| Conservation areas and other Article 2(3) land | Single-storey rear extension still allowed within the limits above, unless an Article 4 direction removes it; no side extensions or external cladding | Not available at all |
| What you submit | Nothing, though a certificate is advisable | Prior approval application, £249 |
| Council decision period | Not applicable | 42 days |
How prior approval actually works
The council is not deciding whether to permit your extension. It is deciding one narrow question: whether the impact on the amenity of adjoining premises is acceptable. It cannot refuse on design, on overdevelopment, on parking, or because it would rather you built something smaller. The steps are fixed.
- You submit a written description of the proposal, a plan of the site, and elevations showing the extension's depth, height and eaves height, along with your address and email. The fee is £249, plus the Planning Portal service charge of £75.83 plus VAT if you submit through the Portal.
- The council notifies the owners and occupiers of every adjoining property, including those to the rear, and gives them 21 days to object in writing.
- If no adjoining neighbour objects, the council is not permitted to consider amenity impact at all. It confirms prior approval is not required and you may build.
- If an adjoining neighbour does object, the council assesses the amenity impact and may grant prior approval, grant it with conditions, or refuse.
- The whole process must conclude within 42 days of a valid application.
The fourth step is why objections are worth taking seriously and the third is why many schemes sail through. A neighbour who dislikes the look of the extension has no route to stop it. A neighbour whose only ground floor window would be enclosed by a 4 metre wall two metres away does.
What happens if the council misses the 42 days
Prior approval is deemed granted and you may proceed exactly as described in your application. This is the single most valuable feature of the route and it has no equivalent in the ordinary planning system, where a missed 8 week deadline gives you only a right of appeal. Diarise the 42nd day from validation, and keep the council's acknowledgement email, because it is your evidence of when the clock started.
Build only what you notified. The deemed approval attaches to the described scheme, so a metre of extra depth added on site is unauthorised development, and since 25 April 2024 that remains enforceable for 10 years.
The limits that catch people out
- Article 2(3) land. The scheme is unavailable in conservation areas, National Parks, areas of outstanding natural beauty and World Heritage Sites. Given how much of inner London sits in a conservation area, check first.
- Flats, maisonettes and listed buildings. Excluded entirely. Class A applies to a dwellinghouse, and a flat is not one.
- The 50 per cent curtilage rule. Buildings and extensions must not cover more than half the area of land around the original house. Existing outbuildings and previous extensions count.
- Side elevations fronting a highway. The extension must not extend beyond a side wall that fronts a road.
- Materials. They must be similar in appearance to the existing house.
- Removed rights. An Article 4 direction, or a condition on an earlier permission, can strip Class A rights altogether. Estates built in the last few decades very often have such a condition. A proposed Lawful Development Certificate is the way to settle the point in writing.
Notify the council once the extension is complete. If any limit is breached, the fallback is a full householder planning application at £548, which is decided in 8 weeks on planning merit rather than on neighbour amenity alone.
Building regulations still apply
Prior approval settles the planning side only. The extension still needs building regulations approval for structure, foundations, insulation, ventilation and drainage, plus a designed steel beam where the rear wall is opened up. If a public sewer runs within 3 metres of the footprint you will also need a Thames Water build over agreement, and if the extension is within 3 metres of a neighbouring structure and deeper than its foundations, a party wall notice.
For costs, see our extension cost guide. GS Renovation coordinates the prior approval application with your designer and carries out the full build across London and the surrounding counties. For a free assessment, contact us or call 07472 424 226.