Permitted development rights for a conservatory don’t just depend on its own size — they also depend on how much of your garden is already covered by buildings, because the total coverage of the plot is capped at 50%.

What the 50% rule actually says

Permitted development for house extensions and conservatories sits under Class A of Part 1, Schedule 2 of the Town and Country Planning (General Permitted Development) (England) Order 2015. One of the specific conditions in Class A is that development isn’t permitted if the total area of ground covered by buildings within the curtilage of the house (other than the original house itself) would exceed 50% of the total area of the curtilage, excluding the ground area of the original house. In plain terms: once buildings other than the original house cover more than half your garden’s total area, permitted development rights for further building work stop applying, and planning permission is needed instead.

What counts toward the 50% limit

The calculation covers rear and side extensions, conservatories, garages (whether attached or detached), garden rooms, sheds, greenhouses, workshops and summer houses, and essentially any other roofed structure within the curtilage. It doesn’t include the original house’s own footprint, and it generally doesn’t include things like patios, driveways, hard standing, fences, walls, or open-sided structures without a solid roof. This means the 50% figure is specifically about roofed buildings occupying garden space, not about how much of the garden is paved, decked, or otherwise hard-landscaped.

Why a previous owner’s extensions still matter

This is the detail that catches people out most often when planning a new conservatory: the 50% allowance doesn’t reset when a property changes hands. Every extension, shed, garage, garden room or other roofed structure built on the plot since the original house was constructed (or since 1 July 1948, if the house predates that) counts toward the running total, regardless of who built it. If you’ve bought a house where a previous owner already added a large extension and a substantial garden room, you may have considerably less headroom left under the 50% limit than the size of your own garden might suggest at first glance.

How to actually check your position before designing

Working this out accurately means measuring the total curtilage area, then adding up the footprint of every existing roofed structure other than the original house, and comparing that total against 50% of the curtilage. Because “curtilage” itself has a specific legal meaning (broadly, the enclosed land forming part of and used in connection with the house, not necessarily the entire plot boundary as marked on a map) it’s worth getting this calculation checked, either through a pre-application enquiry with the local planning authority or via a professional, rather than estimating it visually before committing to a conservatory design.

Why “the original house” isn’t necessarily the house as it stands today

The 50% calculation excludes the “original dwellinghouse” from the coverage figure — but that term has a specific, technical meaning: it refers to the house as it was first built, or as it stood on 1 July 1948 if it was already in existence by then, not the house as it currently stands after any previous extensions. This matters because a house that’s already had a large extension added under permitted development doesn’t get to treat that extension as part of the “original” house for the purposes of a later project — the extension itself counts as additional coverage, on top of the genuinely original footprint, when working out how much of the 50% allowance remains.

What happens if a project would exceed it

• If a proposed conservatory would push total coverage over 50%, permitted development rights simply don’t apply to that specific project, regardless of the conservatory’s own size or design otherwise fitting other permitted development limits.
• This doesn’t mean the conservatory can’t be built — it means a full planning application is needed instead, where the council will assess the proposal on its individual merits rather than it being automatically allowed.
• Building without permission where the 50% limit has genuinely been exceeded means the works aren’t lawful permitted development, which can create real problems later, particularly when selling the property.
• A Lawful Development Certificate can be a useful way to get written confirmation from the council that a specific proposal does fall within permitted development limits, including the 50% rule, before work starts — providing certainty rather than relying on your own calculation alone.

FAQs

Does a garden shed count toward the 50% limit?

Yes — sheds, greenhouses, workshops and similar roofed outbuildings all count toward the total, alongside extensions, conservatories and garages, when calculating whether the 50% coverage limit has been reached.

Does a patio or decking count toward the 50% rule?

Generally no — the rule is specifically about roofed buildings covering the curtilage, not hard landscaping like patios, decking (below a certain height) or driveways, which are treated separately under permitted development rules.

How do I find out how much of my garden is already covered?

You’ll need to measure the curtilage and the footprint of every existing roofed structure, or ask a planning professional to do this for you — some local authorities can also advise on this as part of a pre-application enquiry before you commit to a conservatory design.

Sources

This guide draws on the following primary sources, current as of 17 September 2026:

• legislation.gov.uk: Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1
• PD Assessment: The 50% curtilage rule — what counts and how to calculate it

Related guides

Compare Conservatories is an independent guide. We may earn a fee from some links; this never affects what we write. This article is general information, not legal advice on any specific property — always confirm with your solicitor.