Counting back to the original dwellinghouse

Every allowance in the householder permitted development classes is measured against the original dwellinghouse, and that phrase has a fixed statutory meaning. For a building that existed on 1 July 1948, the original dwellinghouse is the house as it stood on that date. For a building constructed after that date, it is the house as built. So an extension added by a previous owner in 1985 eats into your allowance, a conservatory counts, and a garage conversion may or may not depending on what it was. Work out the baseline before you measure anything else.

This is the single most common reason a householder project turns out not to be permitted development after all.

What original means in Part 1

The interpretation provisions of the General Permitted Development Order 2015 define original by reference to the 1 July 1948 date. The practical effect is a fixed footprint that does not move however many times the house changes hands.

Three consequences flow from it.

Cumulative limits bite. Class A limits the depth, height and position of an enlargement, and several of those limits are expressed by reference to the original dwellinghouse rather than to the house as it now stands. A side extension, for instance, is limited by reference to the width of the original dwellinghouse, not the current width.

Curtilage coverage is cumulative too. The classes covering extensions and outbuildings both contain a test on the total area of ground covered by buildings within the curtilage, other than the original dwellinghouse, which must not exceed half the curtilage area. A large garden room built ten years ago reduces what is left for an extension today.

And permission does not reset anything. An extension built with express planning permission still counts towards the original dwellinghouse allowances, because the test is a comparison with the original house rather than a record of how previous work was authorised.

The 1 July 1948 date and older properties

For older houses, the date works in the owner's favour more often than not. A Victorian terrace extended with a rear outrigger in 1910 and a scullery in 1935 has all of that as part of its original dwellinghouse, because it was there on 1 July 1948.

So the research question for a pre-war house is narrow: what has been added since 1948. For a house built in the 1970s the question is everything since construction, which is usually easier to answer because the planning record exists.

Rebuilt houses are the awkward case. Where a dwelling has been demolished and replaced, the original dwellinghouse is the replacement as built, which resets the baseline. Where a house has been substantially altered but not rebuilt, the original remains the pre-1948 or as-built form, and the alterations count against the allowance.

Finding evidence of earlier extensions

Five sources, in roughly the order they are worth checking:

1. The authority's online planning register, searched by address and by neighbouring addresses, since older applications are sometimes indexed oddly.
2. Historic Ordnance Survey mapping at large scale, which shows building footprints and dates the surveys. The 1940s and 1950s editions are the ones that matter for the 1948 test.
3. Historic aerial photography, which is often available through local studies libraries and county record offices.
4. Title documents and deeds, which sometimes recite earlier works and nearly always contain a plan.
5. Building control records, which may survive where a planning application does not, because many extensions were approved under building regulations and were permitted development.

Where the record is incomplete, a statutory declaration from a previous owner or a long-standing neighbour is acceptable evidence, and councils do accept it where it is specific about dates and extents. Vague recollection does not help.

Photograph what you find and keep it in one file with a short note of where each item came from. That file is what an officer reads.

Extensions built under previous owners

A buyer inherits the allowance as it stands, not a fresh one. That is a point worth raising at the point of purchase rather than at the point of design, because it affects what the property is worth to someone who intends to extend.

Two practical checks on acquisition. Ask the seller's solicitor for the planning history and for any building regulations completion certificates. And compare the current footprint against the earliest mapping you can find, rather than taking the estate agent's description of what is original.

Where previous work was unauthorised and is now immune from enforcement through the passage of time, it still counts towards the original dwellinghouse allowance. Immunity from enforcement is not the same as the work never having happened.

Conservatories and whether they count

They count. A conservatory is an enlargement of the dwellinghouse, and it has been treated as such for a long time. There is no exemption for a lightweight or glazed structure attached to the house.

Where it sits matters as much as whether it counts. A rear conservatory consumes rear extension depth. A side conservatory consumes the side extension width allowance measured against the original house. And a conservatory removed to make way for a new extension does not restore the allowance in itself, because the measurement is of what exists after the proposed work, not of what has ever been built. If the conservatory is demolished as part of the project, say so on the drawings and show the resulting footprint.

Porches have their own small class with its own limits and are assessed separately rather than against the original dwellinghouse allowance. Read that class on its own terms.

Outbuildings and curtilage coverage

The coverage test catches people with large gardens as well as small ones, because it is a proportion rather than an absolute area.

Measure the curtilage boundary carefully, excluding anything that is not within it, and exclude the ground area of the original dwellinghouse from the calculation. Then add up the ground area of everything else: extensions, garages, sheds, garden rooms, greenhouses and hard-roofed stores. Driveways and patios are not buildings and do not count towards coverage, though they have their own class and conditions.

Two traps. A detached garage built in 1960 counts, even though nobody thinks of it as an extension. And a structure the vendor described as temporary counts if it is a building in fact.

Proving the baseline for a certificate

Where the project is finely balanced, do not guess. An application for a certificate of lawfulness of proposed use or development under section 192 of the Town and Country Planning Act 1990 asks the authority to confirm that the proposal would be lawful, and a lawful development certificate is the only formal way to confirm that works are lawful.

What the application needs is a clear statement of the original footprint with evidence, a schedule of additions since the relevant date with dates and areas, a drawing showing the original dwellinghouse, the existing building and the proposal on one plan, and a calculation against each relevant limit in the class. A short Planning Statement tying those together is usually the whole submission.

Two final points. Conditions on an earlier permission may have removed permitted development rights altogether, which is common on estates built since the 1990s, so read the decision notice as well as the Order. And buildings containing flats have no Part 1 rights at all, so none of this analysis applies to a flat above a shop or to a maisonette. Front elevation work raises separate issues again, as set out in why front extensions are harder than rear ones.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Enter your postcode on PlanScope to see which documents your extension will need, including where a certificate is the better route.

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