Walkthrough: upward extension to add flats

Adding two storeys of flats to an existing block can be done under Class AA of Part 20 of Schedule 2 to the General Permitted Development Order 2015, which turns what would be a full planning application into a prior approval. The authority cannot refuse the principle. What it can do is refuse approval on the matters the Order lists, and that list is long enough that a poorly prepared submission fails on its merits anyway.

This is an illustrative walkthrough of one such scheme on a 1970s block, with the sequence and the documents that made it work.

The existing block and whether it qualifies

The building in this example is a detached, purpose-built block of flats of four storeys, completed in 1974, outside any conservation area and not listed.

The eligibility tests under Class AA are specific, and each one is checked before any design work starts.

1. The building is a purpose-built detached block of flats, constructed between 1 July 1948 and 5 March 2018.
2. The proposal adds no more than two additional storeys, and the resulting building does not exceed the height limits set by the class, measured both overall and above the existing roof.
3. The site is not on land excluded by the Order, which rules out conservation areas, National Landscapes, National Parks, World Heritage Sites, listed buildings, scheduled monuments and various safeguarded areas.
4. The new flats will comply with the nationally described space standard, which is a condition of the class rather than a matter of local policy.

Getting these wrong is fatal, because a prior approval granted for development that was never permitted development is worth nothing. Where any test is marginal, a certificate of lawfulness is the only formal way to confirm the position.

What the prior approval actually covers

The authority determines whether prior approval is required for a defined list of matters, and for Class AA that list includes transport and highways impacts, air traffic and defence asset impacts, contamination and flood risk, the external appearance of the building, the provision of adequate natural light in all habitable rooms of the new flats, the impact on the amenity of the existing building and of neighbouring premises including overlooking, privacy and loss of light, and, in London, the impact on a protected view.

Part 20 also requires a construction management plan to be submitted for approval, covering hours of operation and how noise, dust, vibration and traffic effects on the occupiers of the building and on neighbours will be managed.

The authority has eight weeks. The fee is set per new dwelling under the national fee regulations and is indexed, so take the current figure from the published schedule rather than from an older article.

Structural capacity and the survey that proves it

Nothing else matters if the frame cannot take the load. The first spend on this scheme was an intrusive structural survey: opening up to confirm the frame type, trial pits to establish foundation depth and bearing, concrete testing, and a review of the original drawings where they survived.

The result shaped the design more than the planning brief did. Two additional storeys in lightweight steel frame with a modular floor cassette, rather than the concrete originally assumed, because the foundations could take the lighter option without underpinning.

Where the survey shows the foundations cannot cope, the scheme usually stops there. Underpinning an occupied block to add two storeys is rarely affordable.

Daylight, amenity and the existing occupiers

Two amenity questions run in parallel and they are frequently conflated.

The first is the natural light reaching the habitable rooms of the new flats, which is a prior approval matter in its own right. A daylight analysis of the proposed layouts, showing that each habitable room achieves adequate natural light, is needed rather than an assertion that the windows are large.

The second is the effect on existing occupiers and neighbours: overlooking from new balconies, loss of light to properties opposite, overshadowing of gardens. A daylight and sunlight assessment following the national good practice guidance addresses that, and it is the document that answers the objections that arrive from the flats across the street. The way those arguments are weighed is visible in a walkthrough of a householder appeal decided on daylight.

Residents of the block itself are a third constituency. They are not statutory consultees in the way neighbours are, but their objections arrive anyway, and the lease and management position sits behind them.

External appearance and the design submission

External appearance is a prior approval matter, so design is on the table. The submission included elevations of every face, a materials schedule, a bay study at 1:20 showing the junction between the existing brickwork and the new construction, and views from three points in the street.

The junction detail is where these schemes are won or lost visually. A new storey set back behind a parapet reads as an addition. One built flush in a material that almost matches reads as a mistake. Pick one approach and commit to it.

Construction above occupied homes

Residents stay in the building throughout. That single fact drives the construction management plan and most of the programme cost.

The plan submitted covered a crane position and oversail arrangements, working hours restricted around the residents' day, a scaffold design that kept every entrance and escape route usable, temporary weather protection during roof removal, dust and noise monitoring with agreed limits, and a named contact for residents with a published response time.

Write it as though a resident will read it, because one will. A plan that states the hours and the mitigation in plain terms defuses more objections than a technical annex.

Fire safety and the higher-risk threshold

Adding storeys can push a building past the higher-risk threshold of 18 metres or seven storeys, which brings it into the building safety regime with its gateway regime during design and construction.

Check the resulting height early. The consequences are substantial: a different regulatory route under the Building Regulations, additional competence requirements, and a programme measured in months rather than weeks for approvals.

Even below the threshold, the fire strategy for the existing block has to be revisited. Means of escape from a stair designed for four storeys, the fire main provision, compartmentation at the new junction and access for firefighting all change when two storeys are added on top. That work belongs to a fire engineer, and an AI-assisted draft is a starting point for that professional rather than a substitute for their judgement.

The pack that went in

• The prior approval application form and the fee for the number of new dwellings.
• Existing and proposed plans, elevations and sections, with the height measurements marked against the class limits.
• A design and materials statement covering external appearance.
• Daylight analysis for the new habitable rooms, and a daylight and sunlight assessment for neighbouring property.
• A construction management plan.
• A transport note covering parking, refuse and servicing during and after construction.
• A flood risk and contamination screening note, short where the site raised no issues.

The scheme also became liable for the community infrastructure levy, since development under a general consent that creates new dwellings can be chargeable and requires a notice of chargeable development before commencement. Missing that notice is an expensive administrative error, and it is easy to miss precisely because there is no planning permission to prompt it. The parallel prior approval route for commercial to residential conversion is set out in a walkthrough of a shop to residential conversion under Class MA.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Browse the report types on PlanScope to see which supporting documents an upward extension needs. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.