Which reports a rooftop extension to flats needs
Upward extensions to blocks of flats sit between Part 20 prior approval and full planning. Here is the evidence base each route needs, and what changes at 18 metres.
The reports needed for a rooftop extension to flats are driven by three things: whether you are going through prior approval or full planning, whether the finished building reaches 18 m or seven storeys, and how much daylight the new massing takes from the people already living there. A two-storey addition to a 1960s block usually needs a Planning Statement, a Design and Access Statement, a daylight and sunlight assessment, a structural appraisal, a construction management plan and, above 18 m, fire safety information prepared by someone with fire engineering competence. Everything else depends on the site.
Permitted development for extra storeys, and where it stops
Schedule 2 Part 20 of the GPDO 2015 allows up to two additional storeys on a detached block of flats of three or more storeys, and one additional storey where the block has two storeys, subject to prior approval. The building as extended must not exceed 30 m. Each new storey has a floor to ceiling limit, and the extension must sit above the existing principal part of the roof.
The exclusions remove most of the interesting sites. Part 20 does not apply on article 2(3) land, so conservation areas, National Parks, National Landscapes, World Heritage Sites and the Broads are out. Listed buildings and scheduled monuments are out. Blocks built before 1 July 1948 or after 5 March 2018 are out. Sites within safeguarding zones and certain designations are restricted.
Prior approval is not a light touch. The matters the authority considers include transport and highways, contamination and flood risk, external appearance, the provision of adequate natural light in all habitable rooms of the new dwellings, the impact on the amenity of the existing building and neighbouring premises including overlooking, privacy and loss of light, and protected views. Part 20 has been amended repeatedly, so read the current text of the Schedule before you scope the pack rather than working from a summary.
Where the block sits on article 2(3) land or the numbers do not work, it is a full application, and the evidence base grows.
Fire safety information once the building reaches 18 metres
Article 9A of the Development Management Procedure Order 2015 requires a Fire Statement with applications for planning permission for development involving a relevant building. A relevant building is one containing two or more dwellings, or educational accommodation, that is 18 m or more in height or has seven or more storeys. Adding two storeys to a six-storey block is the classic way to cross that line without noticing.
The Building Safety Regulator, within the Health and Safety Executive, is the statutory consultee at Planning Gateway One. The statement has to be submitted on the published form and address matters such as site layout, access for fire appliances, water supplies for firefighting, and the principles of the emergency evacuation strategy in so far as they affect the planning application.
This is a report where a licensed or competent professional is legally required. The Building Safety Act 2022 framework expects the statement to come from someone with the necessary fire engineering competence, and an AI-assisted draft is a starting point for that professional to work from, never a substitute for them. The same holds for a full Fire Safety Strategy to BS 9991 where the design needs one.
Height is measured to the floor surface of the top storey, ignoring roof-top plant and machinery rooms. Get the measured height confirmed in writing early, because a building at 17.8 m and a building at 18.2 m attract different consultation regimes.
Daylight and sunlight to existing occupiers
New storeys on an existing block rarely harm the host building's own windows. They harm the flats opposite, and on a tight urban street that is where the objections come from.
A daylight and sunlight assessment to BRE 209 is the evidence councils expect. The usual tests are vertical sky component at the centre of each affected window, with the 27 per cent value and the 0.8 times former value as the reference points, daylight distribution behind the window where internal layouts are known, and annual probable sunlight hours for windows within 90 degrees of due south. Amenity areas are tested for sun on the ground on 21 March.
Two practical points. Getting internal layouts for neighbouring flats is often impossible, so the report should say what was assumed and why. And a scheme that fails a numerical test is not automatically refused, because BRE 209 is guidance and the 2022 edition is explicit that the figures are advisory and should be applied with regard to context. A well-argued report that quantifies the shortfall, compares it with the prevailing conditions in the surrounding streets and explains the planning balance will do more than one that reports a pass on a scheme nobody believes.
Where the block casts new shadow over a garden or a play area, an overshadowing study covers the ground-level picture that window-based tests miss.
Structural capacity and what planning assesses
Planning does not assess structural adequacy. Building Regulations Part A does, and the structural design is checked at building control, not by the case officer.
That does not make a structural appraisal pointless at application stage. Councils refuse schemes they think are undeliverable, objectors argue that the existing frame cannot take the load, and a short structural appraisal covering the existing frame, foundations and likely load path removes the argument. It also tells you early whether the scheme needs a lightweight frame, which changes the massing, which changes the daylight results.
For a block of any age, commission the appraisal before the planning drawings are frozen. Discovering a transfer structure requirement after validation is expensive in a way that is hard to explain to a client.
Construction access on an occupied block
Residents stay in the building. Everything is craned or hoisted over their heads, scaffolding wraps the flats they live in, and the council knows it will receive complaints. A construction management plan and a construction traffic management plan that deal with crane oversail, hoarding, working hours, dust suppression, out-of-hours deliveries and a named resident liaison route will often be conditioned anyway, so drafting them early is efficient rather than generous.
Where the street is narrow, the highway authority will want swept path drawings and a suspension strategy for parking bays. On a scheme that also crosses the 1,000 square metre threshold, other requirements start to bite at the same time.
Space standards, lifts and refuse for the new homes
The new flats are assessed as new homes, not as an extension. That means the Nationally Described Space Standard where the local plan has adopted it, accessible and adaptable dwelling requirements under Part M where local policy applies them, private amenity space policy, cycle parking and refuse storage.
Lifts are where rooftop schemes die quietly. An existing lift that terminates two floors below the new top storey has to be extended, which usually means a new overrun and machine space, which changes the roofline again. Refuse is the other one: adding eight flats to a bin store sized for twenty-four rarely works, and the waste team's comments arrive late in the determination period.
Townscape, views and the rest of the pack
Added height changes the skyline. Where the building is visible from a conservation area, a listed building's setting or a protected view, a Townscape and Visual Impact Assessment with accurate visual representations is the proportionate response. In London, tall building policy in the London Plan sets out how visual, functional, environmental and cumulative impacts are assessed, and boroughs identify locations where tall buildings are appropriate.
Round out the pack with an energy statement where local policy requires one, an overheating assessment for top-floor flats with large glazed areas, and a wind microclimate assessment on taller or exposed schemes. Check the council's local validation list before you submit, because it governs what gets registered.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Permitted development rights can only be confirmed formally through a Lawful Development Certificate.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. You can browse all 66 report types by category to scope the pack before you start drawing.
Frequently asked questions
Do I need a Fire Statement for a rooftop extension?
You need one with an application for planning permission where the finished building contains two or more dwellings and reaches 18 m in height or seven storeys. Article 9A of the Development Management Procedure Order 2015 sets the requirement and the Building Safety Regulator is consulted at Planning Gateway One. The statement must be prepared by someone with fire engineering competence.
Can I add two storeys under permitted development?
Schedule 2 Part 20 of the GPDO 2015 allows up to two extra storeys on a detached block of flats of three or more storeys, subject to prior approval and a 30 m overall height limit. It does not apply on article 2(3) land, to listed buildings, or to blocks built before 1 July 1948 or after 5 March 2018. A Lawful Development Certificate is the formal way to confirm the position.
How is building height measured for the 18 metre threshold?
Height is taken to the floor surface of the top storey of the building, measured from ground level, and roof-top plant rooms and machinery spaces are disregarded. Because the threshold changes which consultees are involved and which documents are required, it is worth having the measured height confirmed by the design team in writing before the application is scoped.
Does a daylight assessment have to show a pass?
No. BRE 209 is guidance, and the 2022 edition says its numerical targets are advisory and should be applied with regard to context, including the prevailing pattern of development in dense urban areas. A report that quantifies any shortfall, compares it with surrounding conditions and sets out the planning balance is more useful to a case officer than a forced pass.
Does biodiversity net gain apply to a rooftop extension?
Development permitted by a development order is outside the biodiversity gain condition, so a Part 20 prior approval is not caught. A full planning application may be, though small exemptions exist where on-site habitat is not affected beyond the de minimis thresholds. Check the current exemption rules with your local planning authority before assuming the scheme is out of scope.
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.