Extending a house that has already been extended
Extension allowances are measured against the house as it stood in 1948 or as first built, so work by previous owners counts against you. Here is how to establish the original footprint and calculate what remains.
Permitted development for house extensions is measured against the original dwellinghouse, not against the house as you bought it. That single definition is why a family who have never altered their home can find they have no permitted development rights left. The Order defines the original dwellinghouse as the building as it stood on 1 July 1948, or as it was first built if that was later, and every extension added since then counts against the allowance regardless of who built it.
So the first job on any extension project is archaeology rather than design. Work out what was there originally, subtract everything added since, and only then decide what is left.
Why the Order measures against the original house
Schedule 2, Part 1 to the General Permitted Development Order 2015 sets its limits by reference to the original dwellinghouse throughout. Rear extension depths, side extension widths, ground coverage and roof volume are all cumulative in that sense.
The wording that does the work appears in the restrictions on Class A: a proposed enlargement is considered together with any existing enlargement of the original dwellinghouse to which it would be joined. Class B does the same for roof space, with a cumulative volume allowance of 40 cubic metres for a terraced house and 50 cubic metres for other houses, counting any previous enlargement of the roof.
None of this depends on who carried out the earlier work, whether it had permission, or how long ago it happened. Permitted development rights attach to the building, not to the owner.
One qualification worth knowing. Where an earlier extension was built under a full planning permission rather than under permitted development, it still counts towards the cumulative limits, because the limits are measured against the original house rather than against the permitted development that has been used.
Working out what the house looked like originally
Four sources answer this, and using more than one is sensible because each has gaps.
Historic Ordnance Survey mapping shows the footprint at successive survey dates, and for a pre-war house the relevant sheet will usually predate 1948. Aerial photography from the middle of the last century onwards shows roof form and outbuildings. The council's planning history, available online for applications from roughly the mid-1970s onwards and on file before that, records what was applied for. And building control records often survive where planning records do not, particularly for works that needed regulations approval but no permission.
For post-1948 houses the question is easier. The original is the house as built, so the original approved drawings, usually held in the council's planning file for the estate, are the definitive answer.
Deeds and title plans help with curtilage boundaries but not with the building footprint, because a title plan shows ownership rather than structures.
Where records are thin, measure and photograph what exists, identify construction periods from materials and details, and set out the reasoning. An honest account of the evidence beats an assertion with nothing behind it.
Extensions by previous owners
This is the part that catches buyers. A 1970s rear addition and a 1990s side infill, neither of which appears in the conveyancing pack, together consume most of the allowance.
Ask the question before exchange, not after. The seller's property information form, the council's online planning register, and a walk round the outside with an eye for changes in brickwork, roof line and window detail will usually reveal the history.
| What to look for | What it usually indicates |
|---|---|
| Change in brick colour or bond at a rear corner | An added rear extension, often unrecorded |
| A flat roof over part of a two storey house | A later addition or a garage conversion |
| Rooflights in a rear slope with a raised ridge | A loft conversion counting against the Class B volume |
| A conservatory on a masonry plinth | Counts as an extension for the ground coverage limit |
| An outbuilding, garage or garden room | Counts towards the 50 per cent curtilage limit |
A garage converted into a habitable room is a special case, because the conversion itself may not have used any extension allowance while the original garage still counts as a building within the curtilage. The wider set of traps there is covered in the note on converting a garage to a habitable room.
The 50 per cent curtilage limit
Both the extension class and the outbuildings class carry the same ground coverage test. Buildings within the curtilage, other than the original dwellinghouse, must not cover more than half the total area of the curtilage, excluding the ground area of the original house.
Read that carefully, because the arithmetic is unusual. The original house is excluded from both the numerator and the denominator. Everything else counts: extensions, garages, sheds, garden rooms, greenhouses on a permanent base, and swimming pool enclosures.
Measure it properly. Take the curtilage area from a scaled site plan, deduct the original house footprint, and then total the ground area of every other structure. On a small terrace with a rear extension and a garden studio, the result is frequently over half before anyone has drawn the new work, and a raised deck or platform can complicate the picture further, as set out in the note on decking, raised platforms and the 300 millimetre rule.
Curtilage is not the same as the registered title. It is the land serving the house, which can be smaller than the title where part of the garden has been fenced off or sold.
Where permitted development has been removed
Three mechanisms take the rights away, and they are easy to miss.
A condition on the original planning permission for the house or the estate, which is common on developments built since the 1980s and often removes Class A and Class E rights entirely. An Article 4 direction covering the area, which typically removes householder rights in conservation areas. And the building type itself, because flats and maisonettes have no householder permitted development rights at all, for the reasons set out in the note on why householder rights do not apply to flats.
Check the decision notice for the estate before assuming anything. The condition is usually short, buried at the end of a list, and worded by reference to the classes rather than in plain English.
Confirming the position
Where the allowance is tight, apply for a Lawful Development Certificate for the proposed works under section 192 of the Town and Country Planning Act 1990. It is the only formal way to confirm that what you intend is lawful, it costs a fee and takes eight weeks, and it is what a purchaser's solicitor will ask for.
Submit the evidence with it: the original footprint from historic mapping or approved drawings, a schedule of every addition with dates, the curtilage calculation, and dimensioned drawings of what is proposed. A certificate application decided on assumptions is a certificate application refused.
If the calculation shows the allowance is exhausted, a householder planning application is the route, and it is often the better one anyway. A short Planning Statement explaining the cumulative history, the design response and the effect on neighbours turns a technically awkward proposal into a readable case.
To see which documents a householder project is likely to need, enter your postcode on PlanScope and answer the guided questions.
This is general guidance, not legal advice, so check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What does original dwellinghouse mean?
The house as it stood on 1 July 1948 if it was built before that date, or as it was first built if later. Every extension added since counts against the permitted development allowances, regardless of who built it, whether it had planning permission, or how long ago the work was done.
Do extensions built by previous owners count against my allowance?
Yes. Permitted development rights attach to the building, not to the owner, and the limits are cumulative against the original house. A 1970s rear addition and a 1990s side infill will together consume most of the allowance, which is why the planning history should be checked before exchange rather than after.
How is the 50 per cent curtilage limit calculated?
Buildings within the curtilage other than the original house must not cover more than half the total curtilage area, excluding the ground area of the original house from that total. Extensions, garages, sheds, garden rooms and permanent greenhouses all count. Curtilage is the land serving the house, which can be smaller than the registered title.
How do I find out what my house looked like originally?
Historic Ordnance Survey mapping for the footprint at successive survey dates, mid-century aerial photography for roof form and outbuildings, the council's planning history online from roughly the mid-1970s and on file before that, and building control records where planning records are missing. For post-1948 houses, the original approved drawings settle it.
How can I be certain the extension is permitted development?
Apply for a Lawful Development Certificate for the proposed works under section 192 of the Town and Country Planning Act 1990. Submit the original footprint evidence, a dated schedule of every addition, the curtilage calculation and dimensioned drawings. It is the only formal confirmation, and a purchaser's solicitor will ask for it.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.