425000/4: What a Planning Reference Number Like This Reveals About Permitted Development Limits

If you have ever searched a council planning portal and landed on a case reference such as 425000/4, you will know the feeling: a string of digits, a scanned decision notice, and very little context about what it actually means for your own home. Reference numbers like 425000/4 are simply the way a local planning authority tags and tracks an individual case through its system. What matters far more to most homeowners and developers in 2026 is not the number itself, but the substance behind it: what was applied for, whether it needed full planning permission or fell under permitted development, and what limits governed the scheme.

This guide uses a reference such as 425000/4 as a jumping-off point rather than a subject in itself. Instead of decoding the digits, we will walk through the permitted development rules that actually determine whether an extension, loft conversion, porch, upward extension or garden structure needs a formal application at all, and where the hard limits sit. These are the rules that decide whether a case ever needed a reference number in the first place, or whether it could have been built without one.

Why the Underlying Rules Matter More Than the Reference

Every planning file, whether it is numbered 425000/4 or anything else, sits within a framework of permitted development rights set out in national legislation. These rights allow certain works to go ahead without a full planning application, provided the proposal stays within specific limits on size, height, position and location. Step outside those limits, even slightly, and the works stop being permitted development and become the subject of a full application, which is when a case reference like 425000/4 gets generated.

Understanding these thresholds before you start design work saves time, money and the risk of enforcement action later. Below are the core limits that most commonly catch people out.

The 50% Curtilage Rule: The Limit Everyone Forgets

One of the most overlooked restrictions under permitted development for house extensions is the overall coverage limit across the whole curtilage, not just the footprint of the new extension.

Under the relevant permitted development class for extending a house, works are not allowed if, once complete, the total ground area covered by buildings within the curtilage (excluding the original house) would exceed 50% of that curtilage. Crucially, this is a cumulative test. It counts:

• Existing extensions built under previous permissions or permitted development
• Proposed new extensions
• Existing outbuildings, garages and sheds
• Proposed new outbuildings, garages and sheds
• Structures built before 1948, if they still count as buildings within the curtilage

The only thing excluded from this calculation is the footprint of the original house itself (the house as it stood on 1 July 1948, or as first built if later). This catches out a surprising number of homeowners who assume that because their proposed rear extension is modest, it will automatically be permitted development. If the garden already has a garage, a summer house and a shed eating into that 50%, a further extension can tip the whole curtilage over the limit and force a full planning application, complete with its own reference number.

Practical tip: before designing anything, measure the whole curtilage and calculate the footprint of every existing building on it (other than the original house). Only then work out how much "coverage" is left before you hit the 50% ceiling.

Conservation Areas and Other Designated Land: Tighter Rules Apply

If the property in question sits within a conservation area, National Park, Area of Outstanding Natural Beauty, the Broads, or a World Heritage Site (collectively known as "article 2(3) land" in planning legislation, though homeowners rarely need the technical label), permitted development rights for extensions are noticeably tighter than elsewhere.

On this type of designated land:

• No side extensions under permitted development. Any extension that goes beyond a side wall of the house is not permitted development and needs a full application.
• No two-storey rear extensions under permitted development. A rear extension of more than a single storey is excluded from permitted development rights, however compliant it might be on size grounds elsewhere.
• No render, cladding or similar exterior finishes under permitted development. Applying stone, artificial stone, pebble-dash, render, timber, plastic or tile cladding to the exterior is not permitted development in these areas, even on a wall that has nothing to do with an extension.

This is why so many planning files in conservation-area streets, whatever their reference number happens to be, relate to relatively modest works. A single-storey side extension, a change of external materials, or a two-storey rear addition that would sail through permitted development on an ordinary suburban plot will usually need full planning permission if the house sits inside a conservation area boundary. Always check the council's conservation area map before assuming permitted development applies.

Porches: Small Structures, Precise Limits

Porches are one of the few genuinely simple permitted development categories, but the limits are exact and worth knowing before ordering materials.

A porch built outside an external door is permitted development only if all three of the following are true:

• The external ground area does not exceed 3 square metres
• No part of it is more than 3 metres above ground level
• No part of it is within 2 metres of any boundary of the curtilage that fronts a highway

Go over any one of these thresholds, whether it is a slightly larger footprint to fit a pushchair and boots, an ambitious pitched roof taken above 3 metres, or simple positioning too close to the pavement boundary, and the porch needs a full planning application. Because porches are cheap and quick to build, this is an area where people often build first and ask questions later; if the council spots a discrepancy on inspection or from a neighbour's complaint, retrospective applications and even removal notices are a real risk.

Loft Conversions, Dormers and the Balcony Trap

Loft conversions are one of the most popular ways UK homeowners add space, but permitted development rights here have firm edges too.

Roof extensions such as dormers are generally permitted development on ordinary sites, subject to volume and design limits, but there are two points that catch people out repeatedly:

1. Balconies are never included. Whatever else a loft conversion achieves under permitted development, it cannot include a roof balcony. If your loft design involves a Juliet balcony, a recessed terrace or any usable outdoor platform at roof level, that element needs a full planning application regardless of how compliant the rest of the dormer is.
2. No loft dormers at all under permitted development in conservation areas and similar designated land. On article 2(3) land, roof extensions and enlargements are excluded from permitted development entirely. Every loft dormer in a conservation area, National Park, AONB or World Heritage Site needs a planning application, however modest the design.

This is a common source of confusion for people who have seen a similar dormer approved (or apparently unremarked-upon) a few streets away, without realising the neighbouring property sits just outside the conservation area boundary while theirs sits inside it.

Adding Whole Storeys: The Upward Extension Route

Since the rules changed in August 2020, homeowners have had access to a genuinely new form of permitted development: adding entire additional storeys on top of an existing house, known in planning circles as an upward extension.

This route allows a detached, semi-detached or terraced house to be enlarged by:

• Up to 2 additional storeys, where the existing house already has 2 or more storeys
• 1 additional storey, where the existing house has a single storey

However, the qualifying conditions are specific and worth checking carefully before assuming your house is eligible:

• The house must have been built between 1 July 1948 and 28 October 2018
• The additional storeys must sit on the principal part of the house, not on a side or rear wing
• The total height of the extended house must not exceed 18 metres
• Each new storey must add no more than 3.5 metres to the height of the house
• For a house that is not detached, the resulting roof height must not exceed the height of the roof of the adjoining property by more than 3.5 metres
• The house must not be a listed building, and the site must not be article 2(3) land (conservation areas, National Parks, AONBs, the Broads or World Heritage Sites are all excluded from this right)

Even where a scheme meets every one of these conditions, this is not an automatic right in the way a small rear extension might be. Upward extensions always require prior approval from the local planning authority before work starts. The council will assess matters such as the external appearance of the building, the impact on the amenity of neighbouring properties (including things like overlooking and loss of light), and the impact on natural light to adjoining homes. In practice, prior approval applications for upward extensions are typically determined within around 8 weeks, and councils can refuse if the impacts are judged un