Evidence base for a rooftop extension adding storeys

The rooftop extension evidence base is unusual because most of it concerns the building you already own rather than the one you are proposing. Structural capacity, the daylight the existing top floor flats currently enjoy, the wind environment at a height nobody has tested, a single staircase designed for five storeys, and the fact that people will be living underneath the works for a year. A scheme that ignores any of those will be refused or will fail at the first technical review.

Adding two storeys to an occupied block is the hardest small project in residential development. The document schedule reflects that.

Permitted development against a full application

Part 20 of Schedule 2 to the General Permitted Development Order 2015 contains upward extension classes, and a purpose-built detached block of flats is covered by Class A of that Part, with dimensional limits on height, storey count and the age of the building.

The evidence burden barely differs between the two routes. The prior approval matters include external appearance, amenity of the existing building and neighbouring premises, overlooking, privacy, loss of light, adequate natural light in habitable rooms and the impact of construction, so you end up producing most of the same documents. Choose the route on whether the scheme fits the limits, and assume the same technical spend either way.

Structural capacity of the existing building

This comes first because it can end the project in a fortnight.

• Original drawings and the construction type, distinguishing in situ frame, cross-wall masonry, large panel system and steel frame. Large panel system construction changes everything and needs specialist appraisal.
• Foundation type and depth confirmed by trial pits, not inferred from the era.
• A ground investigation on the site, giving bearing capacity and settlement parameters for the increased load.
• Intrusive opening up to confirm reinforcement, panel connections and floor construction where records are incomplete.
• A load path from the new storeys through the existing structure to the ground, with the strengthening needed at each level identified.
• Disproportionate collapse requirements at the finished height, which may require work well below the new floors.
• An asbestos refurbishment and demolition survey before any opening up, under the asbestos regulations.

The output that matters commercially is a strengthening cost, floor by floor, and a statement of what has to happen inside occupied flats. Works inside people's homes drive the programme and the decant budget more than anything on the roof.

Lightweight construction for the new storeys is normal for a reason. Steel or cross-laminated timber with a lightweight facade reduces the load enough to make many schemes viable where concrete would not.

Daylight and sunlight to neighbours and to existing flats

Assessment follows the BRE guidance on site layout planning for daylight and sunlight, most recently revised in 2022. The familiar tests are the vertical sky component, with 27 per cent as the reference value, the no-sky line for daylight distribution within rooms, and annual probable sunlight hours of 25 per cent with 5 per cent in the winter months, along with the rule of thumb that a reduction to less than 0.8 of the former value is noticeable.

A daylight and sunlight assessment for an upward extension has to cover three groups and most reports only cover the first two.

Neighbouring buildings, which on an open site may be barely affected. The existing flats in the same block, particularly the top floor, which lose light to new plant, lift overruns, stair cores, flank wall extensions and the balconies of the flats above them. And the new flats themselves, where internal daylight has to be demonstrated because both the prior approval route and the local plan will ask for it.

Do the existing top floor flats properly, with room-by-room results. Those residents will write to the council, and they will be right.

Wind microclimate at the new height

Adding two storeys changes the wind environment at ground level, on any existing roof terrace and on the new balconies. On a site with tall neighbours the change can be disproportionate to the height added, because of downwash and channelling between buildings.

A Wind Microclimate Assessment uses either wind tunnel testing or computational modelling, assessed against established pedestrian comfort and safety criteria, at entrances, on footways, in amenity space and at the new terraces. Comfort criteria differ by activity: a standing area outside an entrance needs a calmer environment than a thoroughfare.

The assessment is usually required where the building becomes materially taller than its surroundings or where terraces are proposed. Where it is not required, a short screening note explaining why is still worth including, because an objector will raise it.

Fire safety and the higher-risk threshold

Eighteen metres or seven storeys is the line. Cross it with a building containing dwellings and you are in the higher-risk building regime: a Fire Statement must accompany the application or prior approval, the Building Safety Regulator is consulted at Planning Gateway One, and the fire statement has to be prepared by someone with fire engineering competence. This is one of the report types where a competent professional is legally required and an AI-assisted draft is a starting point for that person rather than a substitute.

Below the threshold the questions are no less real. Approved Document B was amended to require a second staircase in new residential buildings above 18 metres, with transitional arrangements, so establish where your scheme falls before the design is fixed. An existing block with one stair cannot easily acquire a second one.

The other issues in practice are fire service access and water supply at the new height, sprinkler provision, the evacuation strategy for existing residents during construction, compartmentation of the new floor over the existing top flats, and the external wall construction of both the new storeys and any facade works to the existing building.

Construction logistics above an occupied building

The construction method statement is a planning document on this kind of scheme, not an afterthought for discharge.

1. Craneage. A tower crane oversailing an occupied roof is rarely acceptable, and oversailing a neighbour's airspace needs their agreement. Mobile cranes with road closures, or offsite modular construction lifted in short bookings, are the usual alternatives.
2. Scaffold and hoist positions, and what they do to the existing entrances, parking, bin stores and fire service access.
3. Delivery routing and times, with a holding arrangement so vehicles do not queue on the street.
4. Protection of the existing roof and the flats beneath during removal of the existing roof covering, including the temporary weathering strategy. Water ingress into occupied flats is the single most common failure.
5. Party wall notices under the Party Wall etc. Act 1996 where the block adjoins another building.
6. A construction environmental management plan dealing with noise and vibration under BS 5228, dust, and working hours.

A Construction Management Plan pulling these together is normally conditioned, and offering a detailed version with the application shortens determination. The comparable discipline for works below an occupied building is set out in which reports a basement under a London terrace needs.

Amenity during construction for existing residents

This is the part that turns a technically sound scheme into a contested one. Residents are not objectors to the principle so much as to the year of their lives it will occupy.

Address it explicitly: working hours tighter than the standard condition, no noisy works before 09:00, a named site contact with a phone number, written notice before any works inside a flat, continuity of water, heating and lifts, temporary arrangements for television and satellite reception where aerials are removed, cleaning of common parts, and compensation or decant arrangements where flats become unusable.

Rights to light are a separate civil matter from the planning daylight assessment, and a neighbouring owner with an established right can seek an injunction whatever the council decides. Check the position early on any scheme close to a boundary.

Illustrative example

A six storey 1970s block at 17 metres takes two additional storeys, reaching 23 metres. That crossing of 18 metres brings Gateway One and a fire statement, triggers a review of the single staircase, and adds sprinkler and evacuation lift questions. The daylight assessment finds the fifth floor flats lose light to the new lift overrun, which is then relocated. The figures are illustrative and each building turns on its own survey.

The document schedule

Expect to produce a Planning Statement, a Design and Access Statement, structural appraisal and ground investigation, a daylight and sunlight assessment covering three receptor groups, a wind assessment or screening note, a fire statement where the threshold is crossed, a noise assessment for new plant, a foul drainage capacity check with the sewerage undertaker, a construction management plan and method statement, a refuse and cycle parking strategy for the increased number of flats, and biodiversity material. On a sealed roof the biodiversity baseline is usually close to zero, so a green roof or planted terrace often delivers the gain, though the exemption position for the route you are using should be confirmed with the authority.

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, and for fire, structural and wind assessment a competent or suitably qualified professional is required.

Browse the 66 report types to see which documents a rooftop extension is likely to need.