The Permitted Development Rules That Catch Everyone Out

The trap: Many homeowners measure their PD allowances against their house as it is now. That is wrong. The GPDO measures everything against the “original house” — the house as it was first built, or as it stood on 1 July 1948 if it was built before that date.

Why it matters: If a previous owner added a conservatory, a side extension, or even a porch, those additions count against your PD allowances. You might think you have room for a 4-metre rear extension, but if a former owner already added a 2-metre conservatory, you only have 2 metres of PD depth left.

What to do: Check your property’s planning history on your local council’s planning portal. Look for any extensions or alterations approved (or built) since the house was originally constructed. If you are unsure, our Permitted Development Certificate Report includes a full constraints check against your specific address.

The trap: The total area of ground covered by buildings within the curtilage of the house (other than the original house itself) must not exceed 50% of the total curtilage area. “Curtilage” broadly means your garden and surrounding land — everything within your property boundary except the footprint of the original house.

Why it matters: Everything counts. Your existing shed, your garage (if it is not part of the original house), that old greenhouse, the previous owner’s conservatory, and your proposed extension all count towards the 50% limit. Many homeowners forget to include existing outbuildings in the calculation.

What to do: Measure the total area of your curtilage (excluding the original house footprint), then add up the footprints of all existing buildings and structures within it. If you are already close to 50%, your new extension or outbuilding may push you over the limit and require planning permission.

The trap: Under Class A, no part of an extension can extend beyond the “principal elevation” of the original house, or any side elevation that fronts a highway. Most people assume the principal elevation is simply the front of the house — but it is not always that straightforward.

Why it matters: If your house has an unusual layout — for example, it sits on a corner plot, faces sideways to the road, or has two elevations that could be considered the “front” — the principal elevation might not be what you expect. Getting it wrong means your extension is not permitted development.

What to do: The principal elevation is generally the elevation that faces the highway and forms the front of the house. But for corner plots or houses with multiple frontages, it can be ambiguous. If there is any doubt, apply for a Lawful Development Certificate before you build — the council will confirm which elevation they consider to be the principal one.

The trap: On designated land (conservation areas, National Parks, AONBs, World Heritage Sites, and the Broads), any extension to a side elevation that fronts a highway is not permitted development at all. Even outside designated land, a side extension cannot exceed half the width of the original house.

Why it matters: If your house is on a corner plot and the side elevation faces a road, the rules are much stricter than for a side elevation facing a neighbour’s garden. Many homeowners on corner plots assume they can extend to the side under PD, only to discover that the highway-facing rule prevents it.

What to do: Check whether your side elevation faces a highway (which includes any road, path, or public right of way). If it does, and you are on designated land, you will need planning permission for any side extension. If you are not on designated land, the half-width rule still applies.

The trap: If your property is in a conservation area, AONB (National Landscape), National Park, World Heritage Site, or the Broads, your permitted development rights are significantly reduced. The restrictions include:

• No cladding of any part of the exterior
• No side extensions under PD
• No rear extensions above one storey
• No roof extensions (dormers) facing a highway
• Outbuildings cannot be placed to the side of the house or between the house and the highway
• Stricter rules on materials, height, and design

Why it matters: Many homeowners do not realise their property is in a conservation area until they check. Conservation area boundaries can be surprisingly tight — your house might be inside one while your neighbour across the road is not.

What to do: Use our free PD checker, which flags conservation areas and other designations automatically. If your property is in a designated area, check the specific GPDO class you are relying on — the restrictions vary by class.

The trap: An Article 4 direction is an order made by your local council that removes specific permitted development rights in a defined area. If your property is covered by one, work that would normally be PD requires planning permission instead.

Why it matters: Article 4 directions are not always obvious. They can cover small groups of streets, individual conservation areas, or entire boroughs. The rights they remove vary — some remove all Class A rights (extensions), others only remove specific rights (like porch additions or roof alterations). You cannot rely on general PD guidance without checking for Article 4s first.

What to do: Check with your local council’s planning department, or use our free PD checker which flags known Article 4 directions at your address. Your local authority page also lists all Article 4 directions in your borough or district.

The trap: PD extensions have strict height limits that are measured against the original house:

• The eaves of your extension cannot be higher than the eaves of the original house
• A single-storey rear extension cannot exceed 4 metres in height
• A two-storey rear extension cannot exceed the height of the ridge of the original house
• Within 2 metres of a boundary, the maximum eaves height is 3 metres

Why it matters: On sloping sites, these measurements can be tricky. The height is measured from the natural ground level, not from any raised patio or decking. If the ground slopes, you measure from the highest point of the natural ground adjacent to the extension — which might give you less height than you expected.

What to do: Get accurate measurements of your existing eaves and ridge heights from natural ground level, not from floor level or any raised areas. If the ground slopes, take measurements at multiple points to find the most restrictive.

The trap: A two-storey rear extension must be at least 7 metres from the rear boundary of the curtilage, and no part of the extension within 2 metres of a boundary can have an eaves height exceeding 3 metres.

Why it matters: This rule kills many two-storey extension plans on smaller plots. If your garden is only 10 metres deep, a 3-metre two-storey extension only leaves 7 metres to the boundary — exactly at the limit. A longer garden is needed to go any deeper. And if your extension runs close to a side boundary, the 3-metre eaves limit applies.

What to do: Measure from the rear wall of the original house to the rear boundary. Subtract 7 metres — that is your maximum two-storey extension depth under PD. If you need more, you will need planning permission.

The trap: Part 1 of the GPDO (which covers extensions, loft conversions, outbuildings, porches, and other residential alterations) only applies to “dwellinghouses.” Flats and maisonettes are not dwellinghouses — they have no Part 1 PD rights.

Why it matters: If you live in a flat (even a ground-floor flat with its own garden), you cannot rely on Part 1 PD rights for any building work. You will need planning permission for extensions, outbuildings, or any other alterations that a house owner could do under PD.

What to do: If you live in a flat or maisonette, assume you need planning permission for any external alterations. Check with your local council.

The trap: When a house is granted planning permission (for example, on a new-build estate), the permission can include conditions that remove some or all PD rights. This is common on new housing developments where the density is tight and the council wants to control future alterations.

Why it matters: You might buy a new-build home assuming you have normal PD rights, only to discover that a condition on the original planning permission prevents extensions, outbuildings, or even new fences without a separate planning application.

What to do: Check the original planning permission for your property. You can find it on your local council’s planning application search. Look for conditions mentioning “permitted development,” “Article 4,” or “removal of PD rights.” If in doubt, a Lawful Development Certificate application is the definitive way to confirm your rights.

The trap: Under Class A, the materials used in any extension must be of a “similar appearance” to the existing house. This is not just about colour — it includes the type of material (brick, render, timber, stone), the bond pattern, the mortar colour, and the roof covering.

Why it matters: This condition is often overlooked because it seems like common sense. But “similar appearance” is a legal requirement, not just a design recommendation. If your extension uses visibly different materials, it could technically fail the PD conditions — even if the size and position are within limits.

What to do: Match materials as closely as possible. For brickwork, use the same brick type, bond pattern, and mortar colour. For roofing, match the tile type and colour. If you cannot find an exact match (common with older houses), get as close as you can and document your reasoning.

The trap: If your home was converted from an office, shop, or other commercial use under permitted development (Class O, Class MA, or similar), the planning permission that allowed the conversion may have removed your Part 1 PD rights. Many office-to-residential conversions include a condition stripping PD rights from the new dwelling.

Why it matters: You might assume that your converted flat or house has normal PD rights, but the original Class O or Class MA approval often explicitly removes them.

What to do: Check the approval that converted the building. If it includes a condition removing PD rights, you will need planning permission for any work that would otherwise be PD.

If your project is within PD limits and you want certainty, apply for a Lawful Development Certificate. It costs £274 and gives you a formal legal document from your council confirming that your project is permitted development. This protects you when selling, remortgaging, or insuring your property — and it catches any of these traps before you spend money on building.

Our Permitted Development Certificate Report gives you a full PD eligibility assessment, property constraints check, and application checklist tailored to your address and project — so you can apply with confidence.