Does a conservatory count towards the extension allowance
A conservatory attached to the house is an enlargement under Class A, so it uses the same depth allowance as a brick extension, measured from the original rear wall.
Yes. A conservatory attached to the house counts towards the conservatory permitted development allowance in exactly the same way as a brick extension, because Class A of Part 1 is about enlarging the dwellinghouse and a conservatory enlarges it. Glass walls and a lightweight roof make no difference to the measurement.
That single fact explains most of the trouble people run into when they come to extend a second time, or to replace an ageing conservatory with something warmer.
Why a conservatory is an extension
Class A of Schedule 2 Part 1 to the General Permitted Development Order 2015 permits the enlargement, improvement or other alteration of a dwellinghouse, subject to limits. A conservatory fixed to the house and open to it is an enlargement. The limits on depth, height, eaves and curtilage coverage all apply to it.
There is one special provision. The requirement that exterior materials be of similar appearance to the existing house does not apply to a conservatory, which is why a glazed structure on a brick house is permitted at all.
A freestanding garden room that is not attached to the house is different. That falls under Class E as an outbuilding and counts against the curtilage coverage limit instead, which is set out under the 50 per cent curtilage rule for garden outbuildings.
What original dwellinghouse means
Every depth measurement under Class A is taken from the original dwellinghouse, and original has a defined meaning: the house as it was built, or as it stood on 1 July 1948 if it was built before that date.
It does not mean the house as you bought it. It does not mean the house as it stands today. A 1930s semi that gained a kitchen extension in 1972 and a conservatory in 1998 still has its original rear wall where the 1930s builder put it, and that line is where the tape measure starts.
So the allowance is a budget that was set when the house was built, and every previous owner has been spending it.
The effect on the remaining depth
Single-storey rear extensions under Class A are limited to 3 m beyond the original rear wall for a terraced or semi-detached house and 4 m for a detached house. The larger home extension procedure, which uses prior approval and a neighbour consultation scheme, doubles those figures but is not available on designated land.
| Situation | Remaining allowance under Class A |
|---|---|
| Semi with a 3 m conservatory on the original rear wall | None at that point on the rear elevation |
| Semi with a 2 m conservatory | 1 m of further projection at that point |
| Detached with a 4 m conservatory | None at that point |
| Conservatory on a side elevation only | Rear allowance unaffected, but curtilage coverage reduced |
| Conservatory removed entirely before works start | Measured from the original wall as if it had never existed |
The last row is the one people miss in the other direction. Demolishing the conservatory does not restore an allowance that the conservatory used, because the allowance was never about the conservatory. It is about the distance from the original wall.
Height matters as much as depth. A single-storey rear extension must not exceed 4 m in height, and where it comes within 2 m of a boundary the eaves must not exceed 3 m. Many conservatories sit close to a boundary and fail on eaves height rather than depth.
Conservatories built by previous owners
They count. There is no exemption for work carried out before you bought the house, and no exemption for work carried out before a particular date other than the 1948 rule for the original building.
This catches people buying a house with an existing conservatory and planning to extend the kitchen beside it. The conservatory may have used the whole rear allowance at that point on the elevation, and it will also have eaten into the 50 per cent curtilage coverage limit, which counts all buildings in the curtilage other than the original house.
Check the planning history before you buy if an extension is part of the plan. A conservatory built without permission that has become lawful through the passage of time is lawful, but it still counts towards the allowance.
Replacing a conservatory with a solid extension
This is the most common version of the question, and the answer surprises people. Replacing a 3.5 m deep conservatory on a semi-detached house with a solid single-storey extension of the same footprint is not permitted development, because 3.5 m exceeds the 3 m limit measured from the original rear wall.
The conservatory was lawful when it was built, perhaps under an older version of the rules or with planning permission. Lawfulness does not transfer to the replacement.
Three routes out. Build the replacement within the current limits, which may mean losing half a metre. Use the larger home extension prior approval procedure where the house is not on designated land. Or apply for full planning permission, supported by a Planning Statement explaining that the proposal replaces an existing structure of the same footprint, which is usually a straightforward case on amenity grounds.
Thermal separation and building control
A conservatory is exempt from most building regulations requirements where it is at ground level, below the stated floor area limit, separated from the house by external quality walls, doors or windows, has independently controlled heating, and the glazing meets the safety requirements.
Take out the separating doors and the exemption falls away. The space becomes part of the heated envelope and has to meet the energy efficiency and structural requirements that apply to an extension, which usually means the glazed roof will not comply.
That is a building regulations question rather than a planning one, and the two regimes reach different answers about the same structure. An extension can be lawful for planning and non-compliant for building control, and the problem surfaces at sale.
Measuring it properly
- Find the original rear wall from historic drawings, the deeds, an old Ordnance Survey plan or the pattern of brickwork in the roof space.
- Measure out to the proposed rear face, including any existing extension standing in the path of the measurement.
- Check the height at the highest point and the eaves height where the extension comes within 2 m of a boundary.
- Calculate the curtilage coverage: all buildings other than the original house, as a proportion of the curtilage excluding the footprint of the original house.
- Check for an Article 4 direction, a conservation area boundary, and any condition on the original permission removing permitted development rights.
- If the answer is close, apply for a lawful development certificate rather than guessing. A certificate is the only formal way to confirm that works are lawful.
Where the numbers do not work, a full application with a Design and Access Statement and a decent set of drawings is not a disaster. Replacement of an existing conservatory with a well-designed extension of similar scale is among the easier householder cases a case officer sees.
To see which documents a householder application needs, browse the 66 report types by category or enter your postcode on PlanScope.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Does a conservatory use up my permitted development allowance?
Yes. A conservatory attached to the house is an enlargement of the dwellinghouse under Class A, so the depth, height, eaves and curtilage coverage limits all apply to it. Lightweight construction and glazing make no difference to the measurement, although the requirement for matching exterior materials does not apply to conservatories.
What does original dwellinghouse mean?
The house as it was originally built, or as it stood on 1 July 1948 for houses built before that date. It does not mean the house as you bought it. All Class A depth measurements run from the original rear wall, so extensions added by previous owners have already used part of the allowance.
Can I replace a conservatory with a solid extension of the same size?
Not necessarily under permitted development. If the conservatory projects further than the current Class A limit measured from the original rear wall, a replacement of the same footprint exceeds that limit. The conservatory's lawfulness does not transfer. Options are to build within the limits, use the larger home extension prior approval route, or apply for full permission.
Does demolishing the conservatory restore my allowance?
No, because the allowance was never about the conservatory. Class A measures from the original rear wall of the original dwellinghouse, so removing a later structure does not move that line. What demolition does free up is curtilage coverage under the 50 per cent limit, which counts all buildings other than the original house.
Why does removing the separating doors matter?
A conservatory is exempt from most building regulations requirements only where it is separated from the house by external quality walls, doors or windows and has independently controlled heating. Remove that separation and it becomes part of the heated envelope, so the energy efficiency requirements apply and a glazed roof will usually fail them.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.