19 Killermont St: A Case Study in Working Out What You Can Actually Build

Every UK homeowner researching an extension, loft conversion or upward extension eventually lands on the same problem: the permitted development rules sound simple in a headline, but they change completely depending on the specific plot, the specific building and the specific area it sits in. A good way to understand how this works in practice is to take a single, real-world style address and walk through the process a planning consultant or a diligent homeowner would follow. For this guide, we will use 19 Killermont St as our worked example, not because of any particular planning history attached to it, but because it is exactly the sort of terraced or semi-detached property that raises the questions most UK homeowners actually have in 2026: can I extend, can I add a storey, can I convert the loft, and what happens if I am in a conservation area.

This article does not tell you what has or hasn't been approved at that address. Instead, it uses it as a case study to show you how to interrogate any property, including your own, against the current permitted development framework.

Why a Single Address Reveals So Much About Planning Potential

When you start researching a property like 19 Killermont St, the first thing to establish isn't the extension you want to build. It's the planning status of the plot itself. Two houses that look identical from the street can have completely different development rights because one sits inside a conservation area boundary and the other doesn't, or because one has already used up its permitted development allowance with a previous extension.

This matters because permitted development rights (PD rights) are not a fixed allowance attached to "a house" in the abstract. They are calculated against:

• the property's build date and structure type (detached, semi-detached, terraced)
• whether it sits on "article 2(3) land" (conservation areas, National Parks, AONBs, the Broads, World Heritage Sites)
• whether it is listed
• how much of the curtilage is already covered by buildings
• what has already been built under previous permitted development or planning permission

For a property such as 19 Killermont St, sitting on a street of similar terraced or semi-detached houses, all five of these factors need checking before you spend a penny on drawings.

Step One: Establish the Planning Status of the Property

Before you touch a permitted development calculation, use your local authority's planning portal to check two things.

Is the property in a conservation area? Council websites publish conservation area maps, and most also let you search by postcode or address. This single check changes almost everything about what you can do under permitted development.

Is the property listed, or does it sit within the curtilage of a listed building? Listed status brings in an entirely separate consent regime (listed building consent) that sits alongside, not instead of, planning permission.

For a hypothetical property like 19 Killermont St, this is the first ten minutes of work and it dictates the rest of the project. If the answer to either question is yes, your permitted development rights shrink significantly, as covered below.

Permitted Development Rights: What They Actually Allow

Permitted development rights let you carry out certain kinds of extension and alteration without applying for full planning permission, provided you stay within strict limits. They are set out in national legislation and apply (with local variation) across England, though many of the same principles are mirrored in Scottish and Welsh planning law with different terminology.

Extensions and the 50% Curtilage Rule

One of the most commonly misunderstood permitted development limits is the curtilage coverage cap. Under the rules governing single-storey and rear extensions, development is not permitted if, once the works are finished, buildings other than the original house would cover more than 50% of the curtilage.

This is a cumulative test, not a one-off calculation for the new extension alone. It counts:

• the proposed new extension
• any existing rear or side extensions already built
• garages, sheds and outbuildings, including ones that predate 1948

Only the footprint of the original house itself is excluded. For a property like 19 Killermont St, this means that if a previous owner has already built a garden room, a garage conversion or a large shed, that eats into the 50% allowance before you even design your extension. A common pitfall here is homeowners assuming the 50% limit only applies to "extensions" in the narrow sense, then discovering during a lawful development certificate application that an old outbuilding has pushed them over the threshold.

Practical tip: get an accurate measured survey of the curtilage and every existing structure before you commission extension drawings. It's far cheaper to find out at the sketch stage that you're at 45% coverage than after you've paid for full architectural drawings.

Conservation Area Restrictions

If a property such as 19 Killermont St is confirmed to sit within a conservation area, permitted development rights are cut back substantially. Specifically:

• No side extensions under permitted development. Any extension beyond the side wall of the house needs a full planning application.
• No two-storey rear extensions under permitted development. A rear extension of more than one storey is not permitted development in a conservation area; it must go through the planning application process.
• No cladding or render under permitted development. Re-cladding the exterior with stone, artificial stone, pebble dash, render, timber, plastic or tiles is not permitted development on article 2(3) land, even if it would be allowed on an identical house outside the conservation area boundary.

This is the single biggest surprise for homeowners buying older terraced or Victorian/Edwardian properties in city centres and inner suburbs, which are exactly the kind of streets likely to fall within a designated conservation area. If 19 Killermont St were in such an area, a homeowner planning a single-storey rear extension could likely proceed under permitted development (subject to the other limits), but a matching two-storey rear addition or a side infill extension would need a formal planning application, with all the added time, cost and design scrutiny that involves.

Loft Conversions and Roof Enlargements

Loft conversions are one of the most popular ways to add space without extending the footprint, but the rules are more restrictive than many homeowners expect.

• No part of the enlarged roof may exceed the height of the highest part of the existing roof.
• A dormer or other roof enlargement on a roof slope that fronts a highway and forms the principal elevation of the house is not permitted development. In plain terms: if the dormer would face the street from the "front" roof slope, you almost always need planning permission.
• Rooflights (flat, non-projecting windows set into the roof slope) are generally more flexible and can often be installed under separate permitted development provisions, including on the front slope, subject to projection limits.

Crucially, none of this applies at all if the property is on article 2(3) land. In a conservation area, National Park, AONB or World Heritage Site, roof extensions of any kind, including rear dormers, are not permitted development. A homeowner at an address like 19 Killermont St who assumes a rear dormer is "always allowed under PD" could be planning an unlawful development if the property turns out to be in a conservation area; in that scenario, a planning application is required regardless of which roof slope is involved.

It's also worth noting that Class B permitted development rights never extend to roof balconies.