The 50 per cent curtilage test in Class A explained

The 50 per cent curtilage rule extension test says that buildings within the curtilage of your house, other than the original house itself, must not cover more than half the curtilage. It sits in paragraph A.1 of Class A of Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015, and the same test appears in Class E for outbuildings. Both the numerator and the denominator exclude the ground area of the original dwellinghouse, which is the part most people get wrong, and the curtilage you measure is the historic one rather than the plot you own today.

Where the 50 per cent figure comes from

Class A permits the enlargement, improvement or other alteration of a dwellinghouse, then takes that permission away in a list of limitations at A.1. One of them removes permitted development where the total area of ground covered by buildings within the curtilage, other than the original dwellinghouse, would exceed 50 per cent of the total area of the curtilage, excluding the ground area of the original dwellinghouse.

Read that twice. You are not comparing building footprint to plot area. You are comparing everything except the original house to everything except the original house's footprint.

An illustrative example. Take a curtilage of 400m² with an original house footprint of 80m². The denominator is 400 minus 80, so 320m². The 50 per cent allowance is 160m². A 20m² shed and a 16m² garage already account for 36m², leaving 124m² before the test bites. Few householders come close, which is why this limitation matters most on small urban plots and on houses that have already been extended twice.

What the original curtilage means and at what date

"Original" is defined in article 2(1) of the Order. For a building that existed on 1 July 1948 it means the building as it stood on that date. For anything built later it means the building as first built. So a 1930s semi that gained a rear extension in 1965 and a garage in 1988 has an original footprint that excludes both, and both count towards the 50 per cent.

The Order also fixes the curtilage itself by reference to the same dates for these purposes, so the relevant area is the curtilage as it was when the house was built, or as at 1 July 1948 for an older house. Check the current wording, because the drafting here has been amended over the years.

The consequence is the one that catches people. Buying a 5m strip of your neighbour's garden in 2001 does not enlarge your permitted development allowance, even though your title plan and your fence both moved. The strip is in your ownership and outside the relevant curtilage.

Which structures count towards the covered area

Buildings count. In practice that means anything with a roof: earlier extensions, a detached or attached garage, a shed, a summerhouse, a greenhouse, a garden office, stables, a pool building.

What does not count is anything that is not a building. Hard standing and a driveway, decking at or near ground level, walls, fences and other means of enclosure, an open pergola, a satellite dish. An uncovered swimming pool is usually accepted as not a building, while the plant room beside it is.

Two awkward cases. A carport open on three sides still has a roof and is generally counted. And a structure that remains mobile, such as a caravan parked on a hardstanding, is not a building, though it may raise a separate question about whether a material change of use has occurred.

Measure ground area covered, not internal floor area, and measure to the external face of the walls including eaves projections where they are supported. Our note on outbuildings under Class E and the fifty per cent test deals with the outbuilding side of the same limitation.

Measuring the curtilage on an awkward plot

Curtilage is not defined in the Order and it is not the same as the title boundary. The working test is the land that serves the dwelling for its domestic purposes, so a garden, a drive, a yard and a domestic paddock immediately adjoining are usually in, while a field let for grazing, a separately accessed paddock or an area fenced off and used for a business are usually out.

Three practical rules. Measure from a scaled measured survey rather than a Land Registry plan, because the Land Registry plan shows ownership and is not survey accurate. Where a boundary has moved, map both the current line and the historic line on the same drawing. And where the curtilage is arguable, present the smaller area in your own calculation, since that is the figure the council will test.

Long, narrow plots behind terraces are where the test actually bites. A 5m wide rear garden 18m long gives 90m², and once the original footprint is deducted the allowance can be under 40m².

Corner plots, shared drives and land outside the title

Corner plots look generous and often are not, because the side garden that fronts the second highway may be within the curtilage for this test while being unusable for a building under other limitations in A.1 about extending beyond a side elevation.

A shared private drive is the common dispute. Where the land is held in common ownership with neighbours, or is subject to rights of way for several houses, it is hard to argue it is within the curtilage of any one dwelling. Leave it out.

Land held on a separate title, land subject to an agricultural tenancy, and land the council's planning history treats as a separate planning unit all fall outside. If an earlier permission described part of the plot as a paddock or as agricultural land, that description will be quoted back at you.

Evidence a council will accept for the original position

Councils want documents, not assertions. The usual bundle is historic Ordnance Survey mapping showing the plot at the relevant date, aerial photography from the local record office or the council's own historic imagery, the council's planning history for the address including decision notices and approved plans for every earlier extension, old sale particulars or conveyancing plans, and a current measured survey with the calculation annotated on it.

Set the calculation out as a table on the drawing: each structure, its ground area, its date, and the running total against the allowance. An officer who can check the arithmetic in two minutes will not ask for more.

Where the position is finely balanced, apply for a Lawful Development Certificate for a proposed use or development under section 192 of the Town and Country Planning Act 1990. It is the only formal way to confirm that works are lawful without planning permission, and the fee is half the equivalent planning application fee. A short Planning Statement setting out the measurement logic goes with it. Article 3(5) and the other general limitations in the Order apply on top of this test, and we cover those in Article 3(5), limitations and conditions in the GPDO.

What to do when the figure is already exceeded

Three routes, in increasing order of effort.

Apply for planning permission. The 50 per cent test is a permitted development limitation, not a policy test, so exceeding it does not mean the extension is unacceptable. It means the council decides rather than the Order, and a well-argued householder application on a plot with no amenity or character harm is often straightforward.

Remove something. Taking down a redundant garage or a 1970s lean-to to bring the figure back under the allowance can be cheaper than an application, and demolition of a small outbuilding is usually permitted development in its own right.

Check whether an existing structure is immune from enforcement. Operational development that has stood long enough becomes lawful, and the four-year period for operational development in England was extended to ten years by the Levelling-up and Regeneration Act 2023 for breaches occurring on or after 25 April 2024. Which period applies depends on the date of the works, so verify the position as at the date you are advising. An immune structure still counts towards the 50 per cent, so this route helps with enforcement risk rather than with the arithmetic.

Dwellings created by conversion are a separate trap, because permitted development rights are often restricted or absent, as explained in permitted development rights on a converted dwelling.

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 reports your site needs.