Walkthrough: a roof extension adding two flats

Adding two flats on the roof of an existing block sounds like the simplest form of development there is, and the thing that decides it is almost never the design. It is whether the existing structure takes the load, whether the new homes get adequate natural light, and whether you can build above occupied flats without a construction route that makes everyone's life intolerable for a year. The walkthrough below is illustrative. The block, the numbers and the outcome are invented to show how the evidence fits together, and nothing in it describes a real application or a real council.

Two routes exist for this kind of scheme: prior approval under Part 20 of Schedule 2 to the GPDO 2015, or a full planning application. The choice is worth making deliberately rather than by default.

The block, its height and the policy context

The illustrative block is a detached four storey building of 1962, brick with a flat roof and a lightweight asphalt covering, containing 16 flats under a single freehold. It sits in an outer London borough, outside any conservation area, with no listed building nearby. Behind it is a car park. Opposite, across a 14 m wide street, is a terrace of two storey houses.

The proposal is one additional storey containing two three bedroom flats, with the existing stair extended and the existing lift car replaced and extended to serve the new level.

Policy support for upward extension exists in both national policy and the London Plan, and the borough's local plan carried a policy on optimising the use of existing buildings. None of that settles the application. What settles it is the specific effect on the people already living in and near the building.

Whether the upward extension classes applied

Part 20 Class A permits new dwellings on detached blocks of flats, subject to a long list of limitations. The ones that mattered on the illustrative scheme were these: the existing building must be at least three storeys above ground level, it must have been constructed within the defined date window, the new storeys sit immediately above the topmost storey, there are limits on the height of each new storey and on the total height of the extended building, and the route is unavailable on article 2(3) land, on or within the curtilage of a listed building, and in several other excluded locations.

The scheme qualified on its face. The reasons for choosing a full application instead were practical.

Prior approval under Part 20 is determined against a defined list of matters, including transport and highways, contamination, flood risk, external appearance, the provision of adequate natural light in all habitable rooms, and the impact on the amenity of the existing building and neighbouring premises. That list is narrower than a planning application, which sounds like an advantage and is not always one, because it leaves no room to argue planning benefits that fall outside the list. On a borderline daylight case, a full application lets you put the housing contribution into the balance. A prior approval does not.

The route also carries its own requirements, including a report on construction management and notification of the existing occupiers, so the paperwork saving is smaller than people expect. Community Infrastructure Levy liability arises either way, and the biodiversity gain position for development under a development order should be confirmed before relying on either route.

Daylight to existing flats below and opposite

This is where the illustrative scheme nearly failed, and the geometry explains why. A new storey on a flat roof does not affect windows in the storeys below it. It affects windows across the street, and it affects anything arranged around a light well or a rear projection.

The assessment examined the facing terrace opposite, 14 m away, measuring vertical sky component at ground and first floor windows and the no sky line position in the rooms behind them. With a 3 m storey added to a 12 m building at that separation, the ground floor front rooms opposite fell below the guidance target. The design response was to set the new storey back 2.5 m from the street elevation, which brought the ground floor results back within the retained ratio and turned the setback into the architectural idea for the elevation.

Three further points came out of the Daylight and Sunlight Assessment.

The new flats themselves had to be shown to receive adequate natural light, which is the test Part 20 makes explicit and which a full application reaches through the housing standards policies. Rooflights to the single aspect bedrooms were added for that reason.

The existing top floor flats lost sunlight to their balconies. That was accepted as a limited harm, but it had to be measured and reported rather than ignored.

The sunlight assessment covered only windows facing within 90 degrees of due south, which excluded the rear elevation entirely. Saying so in the report avoided an objection that the rear had been overlooked.

Structural capacity and the survey carried out

The structural work drove the programme and the cost. A Structural Survey of a 1960s block involves opening up rather than observing, and the illustrative scheme needed four separate exercises.

Opening up at roof level to establish the slab construction and the position of load-bearing walls. Intrusive investigation of two internal walls to confirm whether they were loadbearing or partition. Trial pits at two foundation positions to establish depth, width and bearing stratum. And a review of such original drawings as the borough's building control archive held, which in the illustrative case was two sheets and a specification.

The outcome was that the existing cross walls could take one additional lightweight storey but not two, which is why the scheme is one storey and two flats rather than two storeys and four. A light gauge steel frame with a timber cassette floor kept the added load within capacity, and the foundations needed no underpinning. Had the answer gone the other way, the scheme would not have been viable.

Do this work before the design is fixed. A structural answer that arrives after the planning drawings are complete usually means redrawing them.

Lift, refuse and cycle provision for new homes

Adding homes to an existing block brings the whole building's provision into question, and that is usually the most awkward negotiation.

The lift was the controlling item. Extending it to serve the new storey required a new motor room arrangement and a shaft extension, and it was the single largest construction risk because it took the only lift out of service for several weeks in a building with elderly residents. The mitigation offered was a temporary stair lift and a programme of works in sections.

Refuse capacity in the existing bin store was already tight, and two three bedroom flats pushed it over. The answer was a modest extension to the store at the rear, which needed its own drawing and triggered a question about the refuse collection route.

Cycle parking for the new flats went into the undercroft, which required removal of two car parking spaces. In an outer London borough that is a negotiation with the highway officer rather than a given, and the parking survey evidence mattered more than the policy standard.

Construction access over an occupied building

Nothing on this type of scheme generates more objection than the construction phase, and the application is the only point at which it can be controlled.

The illustrative Construction Management Plan dealt with: a crane position in the rear car park with an oversail of the building but not of third party land, scaffold erected from the car park with a protected pedestrian route to the entrance, delivery hours outside the school run, no works before 08:00 or after 18:00 on weekdays, a dust and noise monitoring commitment, and a named site contact with a published number.

The party wall position was straightforward because the block is detached and in single ownership, but work affecting the structure above occupied flats brings its own notice requirements and the leases had to be checked. Programme slippage from notices is common, and the sequence described in a party wall notice delaying a rear extension programme applies at a larger scale here.

The pack submitted and the resident engagement

The illustrative submission comprised a planning statement, a design and access statement, the daylight and sunlight assessment, the structural survey and a structural method statement, a construction management plan, a refuse and cycle storage strategy, a transport statement covering the parking change, a fire statement where the extended building triggered it, and an energy statement for the new dwellings.

Resident engagement was not required and was done anyway. Two sessions in the building's communal area before submission, a single page summary through every door, and a written commitment on lift availability. The objections that arrived were about construction rather than about the principle, which is the best outcome you can expect, and it meant the officer's report dealt with conditions rather than with refusal.

One honest uncertainty. On a 1960s block, the structural answer is never fully known until the roof is open, and an allowance for strengthening discovered during the works belongs in the budget whatever the survey said. Browse the report types that a scheme like this needs before you fix the programme.

This is general guidance, not legal advice, and the walkthrough above is illustrative rather than a record of any real application. Check the position with your local planning authority and take advice from qualified professionals before relying on it.

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