How previous extensions count against your allowance

Permitted development allowances are measured against the original house, not against the house as it stands today. So previous extensions permitted development allowance questions have an uncomfortable answer for many buyers: a rear extension built by an owner in 1994 reduces what you may build in 2026, and it makes no difference that you did not build it, did not know about it, or bought the house with it already there.

The original dwellinghouse means the house as it was on 1 July 1948, or as it was first built if that was later. Everything added since counts, whether it was built under permitted development, under a planning permission, or unlawfully.

What the Order means by original

Two dates do the work. If the house existed on 1 July 1948, the original dwellinghouse is the house as it stood on that date. If it was built afterwards, the original is the house as first built.

A 1960s semi extended in 1985 has an original that is the 1960s house. A Victorian terrace extended in 1930 has an original that includes the 1930 extension, because the works predate the reference date.

This catches new-build estates hard. A house completed in 2019 with a builder's optional sun room already fitted has that sun room counted as part of the original only if it was there when the house was first built. Where the sun room was added as a later variation after completion, it is an enlargement, and the allowance shrinks. The developer's sales brochure will not tell you which.

Background on the definition itself is set out in the note on what original dwellinghouse means in the GPDO.

How earlier works are counted

Three limits are affected.

Rear depth is measured from the original rear wall. If a previous owner built a three metre single-storey rear extension on a semi-detached house, the full three metre allowance has gone, and a further extension of any depth beyond that wall needs permission.

Ground coverage is the second. Under Class E, the total area of ground covered by buildings within the curtilage, other than the original house, must not exceed 50 per cent of the curtilage excluding the original house footprint. Every garage, shed, garden room, greenhouse and pool house counts, however old and whoever built it.

Roof volume is the third. Class B allowances for enlarging the roof are volume-based, 40 cubic metres on a terraced house and 50 cubic metres otherwise, and they are cumulative with any previous roof enlargement. A dormer built in 1998 eats into what remains.

Extensions built under planning permission still count

This is the point most often misunderstood. Obtaining planning permission for an extension does not reset the permitted development clock.

The Order measures against the original dwellinghouse. It does not care whether the intervening enlargement was permitted development, granted on application, or built without any consent at all. An unlawful extension that has become immune from enforcement is still an enlargement of the original house, and it still counts.

What planning permission can do is remove permitted development rights explicitly. Conditions on new housing estates frequently withdraw Part 1 rights for extensions, outbuildings or roof alterations, which is why the decision notice for the original estate permission is a document worth finding before you design anything.

What counts as an extension

Conservatories count. There is no planning exemption for a conservatory, whatever the Building Regulations position on thermal separation, and a conservatory attached to the rear wall is an enlargement of the dwellinghouse.

Attached garages count as enlargements. Detached garages count towards the Class E ground coverage figure instead.

Porches have their own class with their own limits, and a porch within those limits is not counted against the Class A allowance. Where a previous porch exceeds the class limits, it was an enlargement and it counts.

Roof extensions and dormers count against the Class B volume allowance and, where they extend beyond the original rear wall in a way that adds floor area, they may also bear on Class A.

One category does not count. Works that are not enlargements of the dwellinghouse at all, such as a hard surface under Class F or a boundary fence under Class A of Part 2, leave the extension allowance untouched. Nor does an internal reconfiguration that adds no floor area, even a substantial one.

Finding evidence of earlier works

Start with the council's online planning register. It typically reaches back to the 1990s, sometimes earlier, and a search by address will show applications, decisions and often the approved plans.

Then widen the search. Building control records held by the council or a private approved inspector. Historic Ordnance Survey maps, which show building footprints by survey date. Aerial photography. The title plan and any deeds at the Land Registry. Previous sale particulars and estate agent floorplans. Old photographs held by the family or the neighbours.

Measure the house yourself and compare it to what you find. A rear wall that does not match the 1970s OS footprint tells you something happened, even where no paperwork survives.

An illustrative case. A buyer takes on a 1930s semi with a kitchen that runs two metres past the rear building line and a detached garage at the end of a long garden. The register shows nothing before 1997. Historic mapping shows the kitchen footprint present by the 1960s survey, which makes it a post-1948 enlargement consuming two of the three metres available. The remaining metre of depth is what the design has to work within, and the garage takes a slice of the ground coverage figure as well. The house and the dates here are illustrative.

Where the records are incomplete

Councils lose files, and pre-1990 records are patchy everywhere. The position is not hopeless.

Apply for a lawful development certificate for the proposed works under section 192 of the Town and Country Planning Act 1990, supported by the best evidence you have and a statutory declaration from someone with direct knowledge, such as a long-standing neighbour or a previous owner. The authority decides on the balance of probabilities, and a well-evidenced application with dated photographs and a clear measured survey often succeeds.

Do not rely on a verbal opinion from the duty planner. It is not a defence, and a buyer's solicitor will ask for the certificate when you come to sell.

Measuring what is left, and what to do when nothing is

Draw the original house as a separate layer on the survey. Mark the original rear wall, the original side walls, the original ridge, and the curtilage boundary. Then add each subsequent enlargement with its date and consent status.

That drawing answers the question in five minutes and it becomes the core of any certificate application.

Where the allowance is spent, the route is a householder planning application, judged on design, neighbour amenity and local policy rather than on a set of measurements. That is a different kind of argument, and a well-evidenced Planning Statement addressing the council's householder design guidance is what carries it. On designated land, the prior approval route for larger extensions is closed in any event, as explained in the note on the 56 day prior approval deadline.

Enter your postcode on PlanScope to see which supporting documents a householder application is likely to need.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A lawful development certificate is the only formal way to confirm that works are lawful. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.