Reports for a rooftop extension to a block of flats
Adding storeys to a block of flats under Part 20 is a prior approval, not a free pass. The class names the subjects the council will assess, and structure and daylight lead the list.
A rooftop extension to a block of flats usually goes in under Part 20 of Schedule 2 to the GPDO 2015, and Part 20 is not a light-touch route. The class grants the permission, then lists the subjects the council must assess before you can build, and those subjects are the whole argument: structure, daylight, appearance, amenity, transport, contamination, flooding and air quality.
Two documents decide most of these applications. The engineer's report on whether the building can carry the load, and the daylight assessment covering both the existing residents and the new flats. Get either wrong and the prior approval fails on a matter you cannot argue your way out of.
The upward extension routes
Part 20 contains a family of classes covering different building types. The one that applies to a purpose-built block of flats allows additional storeys subject to three hard limits. The number of new storeys, which depends on how many the building already has. The total height of the extended building. And the construction date of the existing building, which must fall inside a stated window.
Each new storey also has a floor to ceiling height cap, tied to the lower of a fixed figure and the height of existing storeys. That single condition kills more schemes than any policy test, because a 1960s block with 2.4 metre internal heights cannot carry generous new floors.
The class does not apply on article 2(3) land, which takes out conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites. It does not apply to listed buildings or land within their curtilage, or to scheduled monuments, and there are further exclusions covering safety hazard areas and various safeguarded zones. Where a listed building is involved, the position changes completely, and the document pack is covered in reports needed where a listed building is affected.
Where Part 20 is unavailable, or where you want more than it allows, a full planning application for additional storeys remains open. That route costs more time and puts design and density fully in play, but it is not constrained by the construction date or the storey caps.
Read the class in force on the day you intend to submit. Part 20 has been amended since it was introduced, and the limits and exclusions are the parts that moved.
The structural report the class requires
The application must include a report from a chartered engineer or other competent professional confirming that the existing building is structurally strong enough to take the additional storeys.
What makes that report credible is evidence rather than assertion. The engineer needs the original structural drawings where they survive, or intrusive investigation where they do not: trial pits to establish foundation type and depth, cores through slabs, and an assessment of lateral stability, because adding two storeys to a 1960s cross-wall block changes the wind loading before it changes the vertical load.
Three findings recur. Foundations are adequate for vertical load but the building relies on the existing roof structure for restraint. Existing columns can take more load only if the new storeys are built in light gauge steel or cross-laminated timber rather than concrete. Or the lift and stair core cannot be extended without work at ground level that is far more disruptive than the new floors.
A Structural Survey sets out the condition of the existing building, and councils usually expect a suitably qualified author, so professional review of any draft is strongly advised. Where foundations or ground conditions are the question, a geotechnical assessment sits alongside it.
Daylight, twice over
The prior approval subjects require the council to consider the impact on the amenity of the existing building and neighbouring premises, which takes in overlooking, privacy and loss of light. Separately they require it to consider the provision of adequate natural light in all habitable rooms of the new dwellings.
So you need both halves of a BRE 209 assessment. Outward: vertical sky component and no-sky line calculations for the windows of existing flats below and in neighbouring buildings, plus annual probable sunlight hours where a main living room window faces within the relevant arc. Inward: daylight provision in each habitable room of the new flats, against the illuminance targets in the current daylight standard.
The inward test is the one that catches rooftop schemes. A new top-floor flat set back behind a parapet, with a single window under a deep soffit, can fail on natural light while passing everything else. A Daylight and Sunlight Assessment run at concept stage will tell you whether the setback and the glazing line work before the drawings are fixed.
Fire safety once the building gets taller
Height is the trigger. Where the completed building meets the height condition for a relevant building under the planning fire safety provisions, the application must be accompanied by a Fire Statement and the Building Safety Regulator becomes a statutory consultee at Planning Gateway One.
Adding two storeys to a building already close to the threshold is exactly the scenario that brings a block into scope. The statement has to address access for firefighting, the evacuation strategy, the water supply, the site layout and the implications of the proposed construction, and it must be prepared by someone with the necessary fire engineering competence under the Building Safety Act 2022 framework.
PlanScope can draft a Fire Statement as a structured starting point. A licensed or competent professional is legally required for this document, and the draft is a basis for that professional to develop and take responsibility for, not a substitute for fire engineering judgement. For specialist technical reports, qualified professional input may be required.
Two further consequences follow from crossing the threshold. Gateway Two applies at building control stage, run by the Building Safety Regulator rather than the local authority. And the evacuation strategy for the existing residents during construction becomes a live question for the council's own housing and environmental health teams.
Construction management in an occupied building
Everyone below you stays in their flat while you build. That reality shapes the construction management plan more than any policy.
The plan needs to deal with:
- crane or hoist position and oversailing, which may require agreements with neighbours
- scaffold design and the loading it imposes on the existing structure
- protection of the existing roof against water ingress while it is open
- maintaining a compliant means of escape from the existing flats throughout
- deliveries and the loss of parking
- temporary services where risers are being extended
Residents will ask about two things: how long, and what happens when it rains. A plan that answers both in plain terms, with a named site contact, defuses most objections. A construction management plan is the document the council conditions, so write it to be enforceable rather than aspirational.
Noise, hours and consent in advance
Construction noise is controlled separately from planning. Section 60 of the Control of Pollution Act 1974 lets a local authority serve a notice imposing requirements on how works are carried out. Section 61 lets you apply for prior consent, agreeing methods, plant and hours before you start.
Taking the section 61 route is worth the effort on an occupied block. It gives you certainty on working hours, it demonstrates good faith to residents, and a consent is a defence against a subsequent section 60 notice on the same matters. Support it with a noise and vibration assessment to the relevant construction noise standard, predicting levels at the nearest flats below.
Where the scheme also affects a noise-sensitive neighbour or introduces new plant, a noise impact assessment addressing the operational phase belongs in the pack as well.
What to submit, and to whom
For a Part 20 prior approval on a block of flats, expect the council to want:
- A plan indicating the site, and drawings of the proposed storeys and the completed building.
- The structural report from a chartered engineer or other competent professional.
- A daylight and sunlight assessment covering existing occupiers and the new flats.
- An appearance and design statement addressing the external effect on the host building.
- A transport and highways report, and a construction management report.
- A contamination and flooding position, plus an air quality note where relevant.
- A Fire Statement where the height condition is met.
Determination runs to eight weeks, and the procedural paragraph in Part 20 sets out what happens if the council does not determine in time, so read it rather than assuming. How the pack scales up for larger residential schemes is set out in what the report pack adds at fifty dwellings.
Browse all 66 PlanScope report types and start a draft to scope the pack for your building.
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
Which permitted development class covers adding storeys to a block of flats?
Part 20 of Schedule 2 to the GPDO 2015 contains the upward extension classes, with the class for detached blocks of flats allowing additional storeys subject to limits on the number of storeys, the total height and the age of the existing building. It is a prior approval route, so the council assesses the subjects the class lists before work can start.
Do I need a structural engineer's report for a Part 20 application?
Yes. The class requires a report from a chartered engineer or other competent professional confirming that the existing building is structurally capable of taking the additional storeys. Councils treat a report that hedges on foundations or lateral stability as insufficient, so the engineer needs access to the original design information or to intrusive investigation.
Can a rooftop extension be refused on daylight grounds?
Yes, on two counts. The prior approval subjects include the impact on the amenity of the existing building and neighbouring premises, which covers loss of light and overlooking, and separately the provision of adequate natural light in all habitable rooms of the new dwellings. A BRE 209 assessment addressing both is the document that answers the point.
Does a taller building need a Fire Statement?
Where the completed building meets the height condition for a relevant building, an application must be accompanied by a Fire Statement and the Building Safety Regulator becomes a statutory consultee at Planning Gateway One. A Fire Statement has to be prepared by someone with the necessary fire engineering competence, and a drafted document is a starting point for that professional rather than a substitute.
Do I own the roof of my block?
Not necessarily. Airspace above a block is usually retained by the freeholder, and leases, rights of light agreements and covenants can all restrict building over it. There are also statutory rights for leaseholders that can be engaged by a disposal of an interest in the building. Take legal advice on the title before you spend money on design.
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.