Rooftop extension on a block of flats under Part 20

This part 20 upward extension example is illustrative rather than a real scheme. A developer acquires the airspace over a detached five-storey block of 20 flats built in 1968, flat-roofed, in a suburban location outside any designated area, and applies for prior approval under Class A of Part 20 of Schedule 2 to the General Permitted Development Order to build two additional storeys containing eight flats. The prior approval was granted with conditions. The three things that nearly stopped it were daylight to the block across the courtyard, the lift overrun, and the residents' solicitors.

None of those is a policy question. That is the character of a Part 20 application.

The block and the qualifying tests

Six checks were done before anything was drawn, and each was evidenced in the submission rather than asserted.

• Detached. The block stands in its own grounds with no attached neighbour, which puts it within Class A rather than one of the terrace classes.
• Three or more storeys above ground level. Five, so satisfied.
• Constructed between 1 July 1948 and 5 March 2018. A 1968 building, evidenced from the authority's records and the original building control file.
• Not on article 2(3) land. The site sits outside the conservation area boundary, confirmed from the policies map at an appropriate scale.
• Not a listed building, not within the curtilage of one, and not within a scheduled monument's curtilage.
• Not within a safety hazard area or a military explosives storage area, confirmed from the authority's constraints layers.

The detached test is the one most schemes fail. A block joined to a neighbouring building by a single-storey link was argued as detached in an earlier scheme nearby and refused, and the developer here paid for a measured survey to put the point beyond dispute.

An Article 4 direction check came next. Some authorities have made directions aimed specifically at these classes, and an application needed only because a direction withdrew the right carries no fee.

Height, the roofline and the plant overrun

Two additional storeys is the maximum for a building of two or more storeys, and the resulting height for a Class A block must not exceed 30 metres. At five existing storeys plus two, the building reached roughly 21 metres, comfortably inside the cap.

The floor-to-ceiling height of the new storeys is also limited, and that is where the arithmetic got tight. The existing frame is a 1960s in-situ concrete structure with a shallow floor zone. Delivering the required ceiling height in the new flats, over a transfer structure and a services zone, left very little slack, and the scheme ended up with a lightweight steel frame and a raised-access floor rather than a deeper build-up.

The lift was the item that nearly broke the application. Extending the existing shaft to serve two further floors needs a motor room or an overrun above the top landing, and that overrun is part of the building for height purposes. The resolution was a machine-room-less lift with a reduced headroom requirement, specified at design stage rather than after approval, and the overrun was shown on the submitted elevations so that the authority could see the full height.

Roof plant was handled the same way. Flues, the ventilation units and the lift overrun were drawn together in a screened enclosure, dimensioned, because an officer who finds unshown plant on a site visit treats the approval as exceeded.

The prior approval matters and what was submitted

The submission addressed each prior approval matter in a separate, numbered section, which is the format that gets a decision in one round.

The external appearance section was the one the officer engaged with most. A set-back at the two new floors, with a lighter material and a recessed top storey, reads as an addition rather than as an extra two floors of the same building, and it also reduced the overlooking and daylight effects. One design move answered three matters.

Daylight and sunlight, and where the loss fell

The instinctive assumption is that the top-floor residents of the host block suffer. They do not, in daylight terms, because the new storeys sit above their windows rather than in front of them. Their complaint is noise and disruption during construction, which is a different matter.

The loss fell on the facing block across the courtyard. The Daylight and Sunlight Assessment modelled four scenarios: existing conditions, a future baseline including a consented but unbuilt scheme two plots away, the proposal on that baseline, and the proposal on existing conditions. Reporting all four is what stopped the figures being disputed.

Several windows on the upper floors of the facing block fell below the absolute benchmark and satisfied the relative test. Two did not. The response was a design change rather than an argument: the top storey was recessed further on the courtyard elevation, which brought both windows back within the relative test, and the assessment was rerun.

Overlooking was handled by removing roof terraces from the courtyard elevation entirely, placing private amenity space on the opposite side, and fitting fixed obscure-glazed screens to the flank returns. Internal daylight to the new flats was the other half of the work, because adequate natural light in all habitable rooms is a specific requirement and a deep-plan flat with one aspect does not satisfy it. The unit mix was reorganised to give every habitable room an external window, which cost one flat.

Construction access and the residents

Building two storeys on top of 20 occupied flats is a logistics problem dressed as a planning one, and the construction management report is where the authority tests whether you have thought about it.

The report covered working hours, a crane position with its oversail, a scaffold design that kept every entrance open, a temporary roof over the existing structure before the roof was opened up, dust and noise controls, a single point of contact for residents and a weekly notice arrangement. It also dealt with the question residents ask first, which is whether their roof will leak.

Two legal matters ran alongside, and neither is planning. Party wall notices were served on the adjoining owner, and the programme had to allow for the response period and a possible award. The way a notice can reset a programme is set out in our note on when a party wall notice stopped a rear extension programme.

The second was the airspace lease. Granting a lease of the roof space can amount to a relevant disposal, which engages the right of first refusal that qualifying tenants have under landlord and tenant legislation. That is a matter for a property solicitor and it has to be resolved before, not after, the prior approval application. A developer with an approval and no enforceable airspace interest has bought nothing.

A Structural Survey and intrusive investigation of the existing frame and foundations were commissioned before the application, because the findings set the number of storeys that was feasible.

Fire safety and the higher-risk building question

At roughly 21 metres, the completed building sits above the 18 metre threshold, which brings it within the higher-risk building definition for the construction phase gateways under the Building Safety Act framework. That changes the approach to means of escape, external wall construction and firefighting access, and it changes the programme.

The existing block had a single stair. Adding two storeys meant a fire engineering exercise on the whole building rather than on the new floors alone, and the answer involved smoke control to the stair and a firefighting lift provision reviewed against the current standards. A fire safety strategy is a fire engineering document and a competent professional is legally required to prepare it. An AI-assisted draft is a starting point for that professional, never a substitute.

Whether the planning fire statement requirement under Article 9A of the Development Management Procedure Order applies to a prior approval application, as opposed to an application for planning permission, was raised with the authority in writing at the outset and confirmed before submission. Settle that question rather than guessing at it.

The decision and the conditions

Prior approval was granted. The conditions covered materials and cladding samples to be approved before the relevant works, development to be carried out in accordance with the approved construction management report, working and delivery hours, the privacy screens to be installed before occupation, the cycle store to be provided before occupation, plant noise limits, and the recessed top storey to be built as approved with no variation to the courtyard elevation.

The levy was payable on the new floorspace. No affordable housing contribution and no section 106 agreement arose, which is the commercial logic of the route, and the exemption of permitted development from the mandatory biodiversity net gain regime was confirmed against the current provisions rather than assumed.

Draft the Construction Management Plan and the supporting documents on PlanScope, then send them for professional review before submission. This is general guidance, not legal advice, and the scheme above is illustrative. The limits in Part 20 are precise and have been amended, so check the current text of the Order and confirm the position with your local planning authority.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.