Balconies, decking and the 300mm raised platform rule
Part 1 Class A gives no rights for a veranda, balcony or raised platform above 300 millimetres, which catches most decking. Here is how the height is measured and what the council will expect.
Balconies have no permitted development rights under Part 1 of the GPDO 2015, and neither does any raised platform more than 300 millimetres high. Class A, which covers extensions and alterations to a house, is not available where the work would include a veranda, a balcony or a raised platform. So a first-floor balcony on the back of a house needs a planning application, and so does a timber deck that sits a foot off the ground, even though a patio laid at ground level would not.
That 300 millimetre figure does a great deal of work in practice, and most householders meet it by accident.
The exclusion in Class A
Part 1 Class A of the Town and Country Planning (General Permitted Development) (England) Order 2015 permits the enlargement, improvement or other alteration of a dwellinghouse, subject to a long list of limits. One of those limits removes the right where the development would consist of or include the construction or provision of a veranda, balcony or raised platform.
The same exclusion appears in Class E, which deals with buildings and other structures within the curtilage of a house. A raised deck beside a garden room is caught by it just as a balcony on the rear wall is.
Two consequences follow. First, a balcony cannot be added to a permitted development extension and remain permitted development, because the extension as a whole would then include a balcony. Second, Part 1 does not apply to flats at all, so a balcony on a flat is a planning application in every case regardless of height.
The 300 millimetre rule and how it is measured
The interpretation provisions for Part 1 define a raised platform as a platform with a height greater than 0.3 metres. There is no tolerance, no averaging and no exception for timber.
Measurement is from the ground to the walking surface, and on a sloping garden the measurement that matters is the highest point of the gap beneath the deck. A deck built level across a garden that falls 600 millimetres from house to boundary will be under the limit at one end and well over it at the other. The whole structure is then a raised platform.
Build-up is the other trap. Joists at 150 millimetres on bearers at 100 millimetres with a 25 millimetre board already reaches 275 millimetres before any levelling packers, and most ground-level systems need a ventilation gap underneath. If the design is anywhere near the limit, dig the perimeter down rather than building up.
Steps are treated differently from platforms. A flight of steps giving access from a door to the garden is not usually regarded as a raised platform, but a landing at the top of those steps can be, particularly where it is wide enough to stand or sit on. Where the landing exceeds 300 millimetres, assume it needs permission.
Decking, patios and hard surfaces
A patio laid at or near existing ground level is not an extension, not a raised platform and generally not development at all. That is why most ground-level paving goes ahead without anyone asking the question.
Permeability still matters for front gardens, where the separate rules on hard surfaces and permeable paving apply to areas between the house and a highway. Rear gardens have no equivalent control under Part 1, though a lead local flood authority may take an interest on a larger scheme.
Where decking is combined with an outbuilding, the curtilage coverage limit also comes into play, and the 50 per cent curtilage rule counts the ground area of buildings and structures rather than paving. A raised deck is a structure for that purpose.
Roof terraces on an extension
Flat-roofed rear extensions invite the question, and the answer is almost always the same. Using the roof of a single-storey extension as a terrace means providing a balustrade and an access door, and the resulting arrangement is a balcony or raised platform. It is excluded from Class A and from Class B, which covers roof alterations.
Councils treat roof terraces as one of the more sensitive householder proposals because the overlooking is at first-floor level and across the rear boundary rather than along it. Expect officers to look for a 1.7 metre privacy screen on any flank facing a neighbour, a setback from the boundary, and in some cases a restriction on the usable area.
A Juliet balcony sits in a different position. Where the works amount to an opening with a guard rail set in the plane of the wall, with nothing to stand on outside it, many authorities take the view that no balcony has been provided and the work is an alteration within Class A. Others disagree, and the distinction has turned on a few centimetres of projection. Ask the council before you order the ironwork.
What a planning application needs
A balcony or raised deck is a householder application. The validation requirements are modest, but the amenity case needs to be made properly, because overlooking is the most common reason these applications are refused.
- Existing and proposed elevations and plans at a stated scale, with the platform height dimensioned from ground level.
- A section through the garden showing levels, which is what the case officer uses to judge the real relationship with the neighbour.
- Window positions on adjoining properties marked on the plans, with distances to the nearest facing window.
- Details of any privacy screen, including height, material and opacity.
- A short statement addressing the development plan's amenity policy and any supplementary design guidance the council has adopted.
A Planning Statement that deals with separation distances, screening and the sequence of use is proportionate for a householder scheme and avoids the officer having to infer your case from drawings.
Illustrative example: a rear balcony 3 metres wide on a 1930s semi-detached house, 12 metres from the rear boundary, with the neighbour's conservatory 4 metres beyond it. The scheme that gets approved typically has angled or obscured screens to both flanks, a reduced depth of around 1.2 metres so that the space works as access rather than seating, and a condition keeping the screens in place in perpetuity.
Conservation areas, listed buildings and Article 4
In a conservation area the Class A exclusion for balconies and raised platforms applies as it does anywhere, so nothing is gained or lost by the designation on that point. What changes is the weight given to the effect on the character and appearance of the area, which the council must pay special attention to under section 72 of the Planning (Listed Buildings and Conservation Areas) Act 1990.
On a listed building, listed building consent is needed for works affecting character, separately from planning permission, and a rear balcony bolted into historic brickwork will be scrutinised on fixing detail as much as on design.
An Article 4 direction can remove other parts of Class A, which matters if the balcony forms part of a wider scheme including an extension or cladding. Check the direction's exact wording on the council's website rather than assuming it covers everything.
Recording the position before a sale
Conveyancing enquiries catch raised decks more often than any other garden structure, because the buyer's solicitor asks when it was built and whether permission was obtained. A measured drawing in the file is not an answer.
The only formal way to confirm that works are lawful is a certificate of lawfulness. Section 192 of the Town and Country Planning Act 1990 covers proposed works, and section 191 covers works already carried out. For a deck built years ago, a section 191 certificate with dated photographs and invoices settles the question in a way that no amount of correspondence will.
If the deck was built within the enforcement period and exceeds 300 millimetres, a retrospective householder application is usually quicker and costs less than arguing about immunity.
Enter your postcode on PlanScope to see which supporting documents a balcony or terrace application on your house is likely to need.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A certificate of lawfulness is the only formal way to confirm that works are lawful. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Is decking permitted development?
Only if the platform is no more than 300 millimetres high, measured from the ground to the walking surface. Part 1 of the GPDO 2015 defines a raised platform as one higher than 0.3 metres, and Class A and Class E both exclude raised platforms. On sloping ground the highest point governs, so a level deck across a falling garden often needs permission.
Do I need planning permission for a balcony?
Yes. Class A of Part 1 is not available where the work would include a veranda, balcony or raised platform, so a balcony on a house needs a householder planning application. Part 1 does not apply to flats at all, so a balcony on a flat needs permission regardless of its size or height.
Can I use the roof of my extension as a terrace?
Not under permitted development. Adding a balustrade and a door creates a balcony or raised platform, which both Class A and Class B exclude. An application is needed, and officers usually look for a privacy screen of around 1.7 metres on any flank facing a neighbour plus a setback from the boundary.
Is a Juliet balcony treated as a balcony?
Often not. Where the guard sits in the plane of the wall and there is no platform to stand on outside, many councils treat the work as an alteration within Class A. Views differ between authorities and the distinction can turn on a small projection, so confirm the position in writing before ordering the ironwork.
How do I prove an old deck is lawful?
Apply for a certificate of lawfulness for existing use or development under section 191 of the Town and Country Planning Act 1990, with dated photographs, invoices and any other evidence of when the deck was built. That is the only formal confirmation, and it answers the conveyancing enquiry that will otherwise stall a sale.
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.