You've bought a house with plans to extend it. Maybe the kitchen is too small, or you want to add a loft bedroom for a growing family. You've looked at the permitted development rules, the project seems to fit - and then you notice the conservatory the previous owner built in the nineties.
Yes, it counts. Any extension or alteration carried out since the house was first built - or since 1 July 1948 if built before then - counts against your permitted development allowance. It doesn't matter who built it. The allowances are tied to the property, not the owner, and they don't reset when the house is sold.
Every building on your plot - extensions, conservatories, garages, sheds, outbuildings - must not cover more than 50% of the total curtilage area. The original house is excluded from this calculation, but everything else is in.
This is cumulative. If a previous owner built a 20 square metre rear extension and a 15 square metre garage, that's 35 square metres already used. Whatever remains of the 50% is what you have left. On a smaller plot, this can be the binding constraint before you even look at depth or height limits.
You inherit whatever the previous owner left you. No reset on sale. No exceptions.
The maximum depth of a rear extension is measured from the original rear wall - not from any existing extension. This is the single most expensive mistake homeowners make with permitted development.
If a previous owner already built a 3-metre conservatory, and you measure your new extension from the back of it, you're actually 6 metres from the original wall - which exceeds the standard limit for a semi-detached or terraced house. The total from the original wall is what counts, not the additional depth beyond the existing structure.
For depth limits by property type, see our full extensions guide.
Loft conversions have their own cumulative limit. You can add up to 50 cubic metres of roof space on a detached or semi-detached house, or 40 cubic metres on a terrace. If a previous owner added a dormer that used 25 cubic metres, you only have 25 remaining.
This catches buyers off guard. A house with an existing loft conversion may have zero remaining roof volume allowance - meaning any further roof work needs planning permission, even if the existing dormer is small. For full loft rules, see our loft conversion guide.
If you're buying a house with plans to extend, the free eligibility check takes about two minutes and accounts for previous work.
Check the planning history on your local authority's planning portal. Any work that required planning permission or a Lawful Development Certificate will be recorded there. But work done under permitted development without an LDC won't show up.
Look at historical maps. The National Library of Scotland has a free historical Ordnance Survey map viewer covering England. Compare the current footprint to earlier editions to see when extensions were added.
Check from the street and the garden. Dormers, side extensions, and rear extensions are usually visible. If a house has a rear extension, a side return, and a loft dormer, it's had significant previous work regardless of what the planning records show.
Ask the seller. If you're buying, your solicitor should request details of all alterations. But sellers don't always know the full history, especially for work done by previous owners decades ago.
Common mistakes that cost money
Assuming a conservatory doesn't count. Conservatories are extensions under Class A. They count toward curtilage coverage and toward the rear depth measurement from the original wall. Removing an old conservatory does free up the allowance it was using.
Measuring depth from the existing extension. Always measure from the original rear wall. If the existing extension already reaches the permitted depth, you cannot add more under PD regardless of how much garden remains.
Not checking the roof. A previous loft alteration - even an internal one without a dormer - may have used some of the volume allowance. Check before designing a conversion.
PD Assessment Tool
A professional permitted development assessment from a planning consultant typically costs £400-600 and takes 2-3 weeks. This tool checks your specific project against every relevant condition - including cumulative limits from previous work - and produces a formal document you can share with your builder, your architect, or your council.
The first step checks whether your property is eligible - about two minutes, completely free. If your property qualifies, the full project assessment is £47.
Content verified against the Town and Country Planning (General Permitted Development) (England) Order 2015 (as amended), Classes A and B of Part 1, Schedule 2 (legislation.gov.uk, revised version) and the government's technical guidance (September 2019). SI 2026/313 confirmed no changes to Classes A or B. This page is for general information only and does not constitute legal advice.
April 2026