We're Foxtail Oak, an oak frame design-and-build firm with a workshop near Abergavenny in Monmouthshire, so most of the extensions and garden rooms we build sit under Welsh rules, within 90 minutes of the workshop; we supply frames anywhere in the UK.
Permitted development means the rights that let you build within set limits without a planning application. The oak makes no difference to any of it: planning looks at what the building is, how big it is, where it sits, and whether your house is on designated land. What follows is the general position, not legal advice. Your council is the only body that can tell you for certain.
This article covers attached extensions and detached garden rooms. Garages have their own rules. Porches are covered on the porch page.
Start with this checklist
Work through these six questions before you spend money on drawings. Each one can take the decision out of your hands.
- Is the building attached to the house, or standing on its own? Attached is an extension and falls under the extension rules. Detached is an outbuilding and falls under the outbuilding rules. They are different sets of limits.
- Is the house in Wales or England? The extension limits differ. Most of what you'll read online is written for England and doesn't say so.
- Is the house listed, in a National Park, a National Landscape or a conservation area, or subject to an Article 4 direction? Your council's planning map shows most of this.
- Has the house already been extended? "Original house" means as it stood on 1 July 1948, or as first built if later. Previous owners' extensions count against your allowance.
- Is the house a flat, a maisonette or on a modern estate? Flats have no permitted development rights for extensions. Modern developments often have the rights removed by a planning condition.
- What will the room be used for? A detached building used as separate living accommodation, with a bedroom and a kitchen, isn't "incidental to the enjoyment of the house" and needs permission whatever its size.
If any of 3, 5 or 6 applies, assume you need an application until told otherwise.
Attached extensions: the limits in Wales
A single-storey rear extension can be permitted development if it comes no more than 4 metres out from the rear wall of the original house, whatever type of house it is. It must be no more than 4 metres high. Where it is within 2 metres of a boundary, the eaves must be no higher than 3 metres. Together with other additions, it must not cover more than half the land around the original house, and nothing can go forward of the principal elevation.
Wales has no "larger extension" route. The 6 and 8 metre figures you'll see online belong to England.
Attached extensions: the limits in England
The same rear extension can be permitted development up to 4 metres on a detached house, or 3 metres on any other house. The 4 metre overall height, the 3 metre eaves limit within 2 metres of a boundary, the half-the-land rule and the principal-elevation rule all apply as in Wales.
England also has a larger-extension route: up to 8 metres on a detached house or 6 metres on any other, outside designated land. It needs prior approval from the council, which consults your neighbours, so it is still an application, even though it isn't planning permission.
The height rule that catches oak frames
The eaves limit within 2 metres of a boundary is the one that catches oak extensions, because an oak frame open to the roof is taller than a flat-roofed extension of the same footprint. A generous frame with a decent roof pitch, sited close to a boundary, breaches the 3 metre eaves limit long before it breaches anything else. If your plot is tight, measure the boundary distance first.
Detached garden rooms: the outbuilding rules
A garden room standing on its own is not an extension. It falls under the rules for outbuildings, which are more generous, and they are close to identical in Wales and England.
In both countries an outbuilding can be permitted development if it is single storey; no more than 4 metres high with a dual-pitched roof or 3 metres with any other roof; eaves no higher than 2.5 metres; no higher than 2.5 metres anywhere within 2 metres of a boundary; not forward of the front of the house; with no veranda, balcony or raised platform over 300 mm; and with all outbuildings and extensions together covering no more than half the garden. It has to be for use incidental to the house, not separate living accommodation.
Wales adds two rules. Nothing over 1.5 metres high within 2 metres of your own house, which catches a garden room tucked in beside it. And a building that goes beyond a side wall of the house must not be nearer a road than the house is, or within 5 metres of it.
That word "incidental" does real work. A garden room used as a home office or a studio is generally fine. One with a bedroom and a kitchen is a separate dwelling, and needs permission.
Designated land, and listed buildings
Designated land and listed buildings are two different things, with different rules, and a house can be both.
Designated land means a National Park, a National Landscape (formerly an Area of Outstanding Natural Beauty) or a conservation area. Bannau Brycheiniog and the Wye Valley both count, and between them they cover a good deal of our working area. On designated land, permitted development for extensions is tighter: single storey only, no cladding, and side extensions limited in Wales and removed in England. For outbuildings, anything more than 20 metres from the house is capped at 10 square metres in total, and nothing can go between a side wall of the house and the side boundary. Article 4 directions, which some councils use to remove rights on specific streets, are not always obvious. Ask.
A listed building is different. Listed building consent is a separate consent from planning permission. It covers work inside as well as outside, and you may need both consents for the same project. Work without it is an offence and never becomes lawful with time. Any outbuilding within the curtilage of a listed building needs planning permission as well. If your house is listed, start with the council's conservation officer before anything is drawn.
FOXTAIL OAK We've built plenty of projects inside conservation areas. We have not, as far as we know, built on a listed building, so we don't claim that experience. If your house is listed we'd bring in an architect from the start rather than guess.
Building Regulations are a separate question
Planning controls whether you can build. Building Regulations control how it's built. You can need one, both or neither.
A heated extension opened up to the house always needs Building Regulations approval: structure, insulation, glazing area, ventilation and safety glass. A conservatory at ground level under 30 square metres, separated from the house by external-quality doors or windows, with safety glazing and the house's heating not extended into it, is usually exempt. Take out the dividing door and it stops being a conservatory in the regulatory sense and becomes a heated extension.
Most of what we build doesn't qualify for the exemption, because most people want the room open to the house. That's worth settling early: it changes the glazing specification, the insulation strategy and the cost, not just the paperwork.
Detached garden rooms follow the outbuilding exemptions instead: under 15 square metres of floor area, exempt outright if nobody sleeps in it; between 15 and 30 square metres, exempt if it is at least 1 metre from any boundary or built of non-combustible materials. An oak frame is combustible, so in practice that means keeping it 1 metre from the boundary. Electrics and drainage need their own approval or a registered installer even where the building is exempt.
Should I get a Lawful Development Certificate?
Yes, if your project is permitted development. A Lawful Development Certificate is a document from the council confirming that no planning permission is needed. It isn't compulsory. It is very useful when you sell the house and a buyer's solicitor asks what authority the extension was built under. "We were told it was fine" is not an answer that survives conveyancing. It covers planning only, not Building Regulations.
FOXTAIL OAK Even when something is permitted development, we advise getting the certificate to prove it, so there are no questions down the line.
Who makes the application?
FOXTAIL OAK On a design-and-build job we develop the design with you first. If it needs planning, we bring in an external architect, usually Ben Bowker at BB Design Services, who prepares the planning drawings, makes the submission and produces the Building Regulations drawings if the project needs them. On a design-and-build job that architect is inside our price. On supply and install, we can arrange the application as an extra. On supply only, you make the application and we'll advise. The council's own planning and building control fees are paid by you and aren't in our prices on any route. If you already have an architect, we work within their design.
We'll tell you on the first call which side of the line we think you're on, and say so plainly if we think you need permission. We'd rather lose the job on that call than take a deposit for a frame that can't be built.
The short version
An attached oak extension on an unlisted house outside designated land is often permitted development if it stays within 4 metres (Wales), or 4 metres detached and 3 metres otherwise (England), keeps under 4 metres high and keeps its eaves under 3 metres near a boundary. A detached garden room has more room under the outbuilding rules. If you're listed, on designated land, in a flat, or on a modern estate, assume you need an application. And if you're close to a boundary, check the eaves height before anything else.
What to do next
Before you talk to us or anyone else, establish three things: where the house is and what designations apply, what the room is for, and what has already been added since 1948. With those, we can tell you on a fifteen-minute call whether planning is likely, and whether it's a project we can do well. Our extension prices and who does what on each route are on the extensions page.
Related questions
Does an oak frame need planning permission that a brick extension wouldn't? No. Planning looks at size, position, height and designation, not the material. The oak matters only because an open-roofed frame tends to be taller than a flat-roofed one of the same footprint.
Does a conservatory count differently from an extension? Not for planning; a conservatory is an extension that happens to be mostly glass. The distinction only matters under Building Regulations, where a separated, unheated conservatory under 30 square metres can be exempt.
What if the house has already been extended? Earlier extensions, including a previous owner's, use up the permitted development allowance. A house extended twice may have very little left, and the only way to know is to check the original house against what stands now.
How long does planning take? The statutory target for a householder application is eight weeks from validation, and validation can take a week or two first. Some come back early, some late, and some drag on when there are many consultees.
Rules checked against the Planning Portal (England) and the Welsh Government's guidance (Wales), 22 September 2026.
Your council has the final word. If you are unsure, ask them, or apply for a Lawful Development Certificate.
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