Measuring the original dwellinghouse for Class A limits

Every measurement in Class A runs from the original dwellinghouse, and the original dwellinghouse is almost never the house you bought. Article 2(1) of the GPDO 2015 defines it as the building as it existed on 1 July 1948, where the building was standing on that date, or as first built where it was built later.

So the three metres a semi-detached house gets for a single-storey rear extension is three metres beyond the 1948 rear wall, not three metres beyond the kitchen the last owner added in 2004. Establish that wall first. Everything else in the design follows from it.

Where the baseline sits

Two rules cover every house in England.

If the house was standing on 1 July 1948, the baseline is its footprint and form on that day. If it was built after that date, the baseline is the house as first completed. Nothing added afterwards forms part of the original, whoever added it and whether or not it had planning permission.

That date is the day the Town and Country Planning Act 1947 brought the modern planning system into force. The Order fixes the baseline there rather than asking anyone to reconstruct an eighteenth-century cottage. For the great majority of English housing stock, built in estates from the 1950s onwards, the baseline is simply the house as the developer completed it.

One consequence catches people out on older properties. A Victorian terrace that was extended in 1930 has an original dwellinghouse that includes the 1930 extension, because that work was already there on 1 July 1948. The house is larger than the one the builder finished in 1890, and the allowance is measured from the 1930 rear wall.

What the baseline is used for

The limits that depend on it are the ones people care about:

• how far a single-storey rear extension may project beyond the rear wall, and the larger figures available under the prior approval route
• how far a two-storey rear extension may project, and the minimum distance it must keep from the rear boundary
• the maximum width of a side extension, expressed as a proportion of the width of the original dwellinghouse
• the total area of ground within the curtilage that may be covered by buildings, where the curtilage is measured excluding the original dwellinghouse footprint
• whether an extension is within a set distance of a boundary, which caps eaves height

The rear extension figures and the prior approval route are set out in detail in Class A rear extension limits and the larger home scheme. Which walls count as rear, side and principal elevation is its own tangle, covered in curtilage, principal elevation and Class A cases.

Extensions by previous owners still count

There is no reset on sale, no reset on a change of council and no reset after a period of time. A rear extension built in 1985 by an owner three sales ago reduces your allowance by exactly the distance it projects.

The same applies to outbuildings under Class E, which count towards the curtilage coverage limit whether you built them or inherited them. A detached garage, a large shed and a home office can together take a garden past the coverage threshold before any extension is drawn.

The arithmetic for combining earlier work with a new proposal is set out in how previous extensions count against your allowance. What matters here is only the starting line.

Finding the evidence

Work down this list in order, because the free sources answer most cases.

1. The council's planning history for the address, searched on its public register. Earlier applications and their approved drawings often show the house before the extension you are trying to date.
2. Historic Ordnance Survey mapping at 1:1250 or 1:2500. Successive editions bracket the date a footprint changed, and for a 1948 baseline the relevant edition is the one surveyed closest to that year.
3. Aerial photography. Post-war survey coverage of England is wide, and a vertical image showing the roof outline is often decisive.
4. The Land Registry title plan and any filed deed plan. The plan is not a measured survey, but a title plan predating an extension is useful corroboration.
5. Building control records held by the council or an approved inspector, which date the works even where no planning application was made.
6. Sale particulars, valuation reports and insurance schedules from earlier transactions.
7. Statutory declarations from neighbours or former occupiers who can speak from first-hand knowledge, giving dates rather than impressions.

Then measure the house. A measured survey of the existing building, with the original walls identified on the drawing and the reason for that identification annotated, is what converts research into something an officer can accept. Changes in brick bond, a break in the damp proof course line, a step in the roof ridge and a different floor level all tend to mark the join.

Rebuilds and conversions

A lawful demolition and rebuild produces a new original, because the building is then "as so built". That sounds generous and rarely is. Demolishing a dwelling and rebuilding it needs planning permission, and the permission commonly carries a condition removing Part 1 rights so that the council keeps control of the finished scheme. Read the conditions on the permission before assuming a fresh allowance.

Conversions are harder. Where a building becomes a dwellinghouse through a change of use rather than by being built as one, the baseline depends on the building as it stood when it became a dwelling, and the position is less settled than for new build. The Order also withdraws Part 1 rights for dwellinghouses created under several of the Part 3 change of use classes, which is why a barn converted under Class Q cannot simply add a rear extension afterwards. The 2024 amendments to that class are covered in Class Q barn conversions and the 2024 amendments.

Flats and maisonettes have no Part 1 rights at all. Class A applies to a dwellinghouse, and a building containing flats is not one.

Recording the calculation

Put the workings in writing before you build, in a form you can hand to a buyer's solicitor in ten years.

A one-page note works: a plan showing the original footprint with the source for each line, a schedule of every subsequent extension with its date and projection, the resulting remaining allowance, and copies of the historic mapping and photographs relied on. Where the proposal comes within a few hundred millimetres of a limit, apply for a certificate of lawfulness of proposed use or development under section 192 of the Town and Country Planning Act 1990. It costs half a planning fee and it removes the argument permanently.

Do not rely on a builder's assurance, and do not rely on a neighbour having done the same thing. The neighbour may have had permission, or may simply have become immune from enforcement.

When the baseline cannot be established

Sometimes the records run out. A house rebuilt after war damage with no surviving drawings, or a much-altered rural cottage with no planning history, can leave you unable to say where the original walls were.

In that position the honest answer is that permitted development is not available to you as a safe route, because the burden of proof sits with the person asserting lawfulness. Two options remain. Apply for planning permission on the merits, which puts the question of the original footprint to one side entirely. Or apply for a section 192 certificate with the best evidence you have and accept that a refusal tells you something useful.

A Planning Statement setting out the site history, the evidence for the original footprint and the policy context is the document that carries either route. Draft a Planning Statement with PlanScope and have it checked before it goes in.

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.