Original dwellinghouse: the date that decides your allowance
Every Part 1 allowance is measured from the original dwellinghouse, not the house you bought. Extensions built by previous owners come out of your budget.
Every permitted development allowance in Part 1 of the General Permitted Development Order 2015 is measured against the original dwellinghouse. The Order defines that as the building as it stood on 1 July 1948, or as first built if it went up after that date. Not the house on the estate agent's floorplan. Not the house you bought.
Once that lands, a lot of householder disappointment makes sense. The 3-metre rear extension a previous owner added in 1998 came out of the same budget you were planning to spend.
The definition in the Order and where it sits
The interpretation provisions of the Order define "original" for the whole instrument, and Part 1 then uses the phrase "original dwellinghouse" throughout its class limits. The government's technical guidance for householders explains the same point in plainer terms and is the document most case officers reach for.
Four separate limits depend on it:
- Rear projections under Class A are measured beyond the rear wall of the original dwellinghouse.
- Side extension width is capped at half the width of the original dwellinghouse.
- Ground coverage is capped at 50 per cent of the curtilage of the original dwellinghouse, counting every building other than the original house.
- Roof volume allowances under Class B are cumulative against the original roof space.
Miss the definition and all four calculations come out wrong in the same direction, which is why extensions get built and then investigated.
The 1 July 1948 line and what it means
1 July 1948 is the day the Town and Country Planning Act 1947 took effect and planning permission became a general requirement. Anything standing on that date was lawful by virtue of existing.
For a Victorian terrace, the original dwellinghouse is therefore the house as it stood in 1948, complete with whatever rear outrigger, scullery or lean-to it had acquired in the previous eighty years. That is often more generous than owners expect, because a two-storey back addition built in 1890 counts as part of the original house rather than as an extension.
For a post-war house, original means as built. A 1972 detached house is measured as it left the builder's hands, and every subsequent conservatory, garage and porch is an addition.
Extensions built by previous owners
The allowance runs with the land. There is no reset on sale, no renewal after a period of years, and no discretion for a council to overlook a previous extension because the current owner did not build it.
| Situation | Effect on a semi-detached house |
|---|---|
| No previous extensions | Full 3 m single-storey rear allowance, or 6 m under prior approval |
| 3 m rear extension added 1998 | Standard rear allowance used up |
| Side extension of half the original width | No further side extension under Class A |
| Detached garage covering 40 per cent of curtilage | Only 10 per cent of ground coverage remains |
The practical consequence for buyers is that a house already extended is often a house that needs a planning application for anything further. Worth knowing before the survey rather than after. And the seller's answers on the property information form are not evidence: they record what the seller believes, which on a house that has changed hands three times since the extension was built is usually a guess.
Evidencing the original footprint
The burden of proof sits with the applicant, and councils will not take a description on trust. The sources that work:
- Historic Ordnance Survey mapping, available through county record offices, larger libraries and subscription map services. The 1940s and 1950s editions are the ones that matter for pre-war houses.
- Aerial photography. Post-war vertical survey photography covers most of England, and archive collections held nationally and by local authorities can date an addition to within a few years.
- The council's own planning history for the address, which shows every application made since records began and often includes the drawings.
- Title deeds and conveyance plans, particularly where a plot was sold off or a boundary moved.
- Building control records, which sometimes survive where planning records do not.
- Sales particulars, insurance surveys and dated photographs from the family album.
Where the paper trail is thin, a statutory declaration from someone with direct knowledge can help, though on its own it rarely carries a marginal case. Councils weigh documents above recollection.
One habit saves repeated trips. Assemble the evidence into a single dated pack with a short covering note explaining what each item shows, and keep it with the deeds. The same pack answers the certificate application, the buyer's solicitor five years later and any enforcement enquiry in between.
Demolished additions and whether they count
This is where practitioners disagree, and the honest answer is that it depends on the limit being tested.
The 50 per cent curtilage test looks at what is on the ground now, so demolishing a large garage does free capacity. The rear projection limits look at the position of the original rear wall, which demolition does not move: knock down a 1990s rear extension and you are still measuring from where the original house ended, so the allowance is available again for a new projection of the permitted depth.
Cumulative roof volume under Class B is the difficult one, because the wording addresses previous enlargements of the original roof space rather than existing ones. Where a dormer has been removed, take advice before assuming the volume has come back.
Illustrative example: a 1930s semi with a 1985 single-storey rear extension of 3 metres and a detached garage. Demolishing both and building a new 3-metre rear extension is likely to be within Class A, since the original rear wall is the reference point and the garage no longer counts towards ground coverage. Building 4 metres is not, because the depth limit does not increase.
Where the allowance has already been used up
Two routes remain, and they are not equivalent.
The larger home extension procedure allows a single-storey rear extension of up to 6 metres on a semi-detached or terraced house and 8 metres on a detached one, subject to prior approval and the neighbour consultation scheme. It is still measured from the original rear wall, so it helps only where the earlier extension was modest or has been removed.
Otherwise it is a householder planning application. That is not a defeat. It costs a fee and around eight weeks, it lets you design the extension the house actually needs, and a short Planning Statement addressing the council's amenity and design policies is usually enough supporting material for a domestic scheme.
Whichever route you take, apply for a lawful development certificate for proposed development under section 192 of the Town and Country Planning Act 1990 where you are relying on permitted development. It is the only formal way to confirm that works are lawful, and it converts an argument about a 1948 map into a document a conveyancer will accept. Set the evidence out in the application rather than waiting to be asked.
To see which supporting documents your scheme is likely to need, browse the report types or enter your postcode on PlanScope.
This is general guidance, not legal advice; 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 in permitted development?
The house as it was first built, or as it stood on 1 July 1948 if it is older than that. The definition sits in the interpretation provisions of the General Permitted Development Order 2015. It fixes the baseline against which rear projection, side extension width, roof volume and curtilage coverage are all measured.
Why 1 July 1948?
That is the date the Town and Country Planning Act 1947 brought the modern planning system into force. Anything built before it predates the requirement for planning permission, so the Order treats the building as it then stood as the starting point rather than trying to reconstruct a Victorian original.
Do extensions built by a previous owner count against my allowance?
Yes. The allowance attaches to the house rather than to the owner, so a 3-metre rear extension added in 1998 uses up the standard single-storey rear allowance for a semi-detached house whatever the deeds say. Buying a house with an extension means buying a house with less permitted development left.
If I demolish an old extension, does the allowance come back?
Partly, and it depends which limit you are testing. Removing an outbuilding frees ground coverage under the 50 per cent curtilage test. It does not move the original rear wall, which is where rear projections are still measured from. Cumulative allowances are the awkward ones, so take advice or apply for a certificate.
Do flats have permitted development rights for extensions?
No. Part 1 of Schedule 2 applies to dwellinghouses, and the Order excludes flats and buildings containing flats from that definition. Anything a leaseholder wants to do externally needs planning permission, along with the freeholder's consent and, in many cases, party wall notices.
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.