Adding storeys to a block of flats under Part 20

Part 20 upward extension of flats under prior approval is a real route to new homes, and it fails more often on title and structure than on planning. Class A of Part 20 permits up to two additional storeys of new flats on a purpose-built detached block, subject to prior approval of a defined list of matters. The planning assessment is narrow. The engineering, the daylight evidence and the rooftop demise are where schemes die.

Before anything else, check three facts: the building is detached, it was built within the qualifying dates, and you control the airspace.

Which buildings Part 20 Class A can apply to

Class A applies to a purpose-built detached block of flats. Detached means exactly that, so a block joined to anything, including by a shared stair core or a linked undercroft, takes you outside the class and into Class AB or into a full application.

The building must have been constructed between 1 July 1948 and 5 March 2018. A block completed after that cut-off does not qualify, and nor does a pre-war building, which rules out a great deal of interwar and Victorian stock.

Designated land is excluded. Conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites are all out, as are listed buildings and buildings in a listed curtilage, scheduled monuments, sites of special scientific interest, safety hazard areas and military explosives storage areas. Check for an Article 4 direction as well, since authorities in London and elsewhere have used them against Part 20.

The resulting storeys must be used as dwellinghouses within Class C3. You cannot add a rooftop office or short-term let under this class.

Height limits and the number of storeys

The limits are cumulative and each one is a hard stop:

• Two additional storeys where the existing building has two or more storeys, one additional storey where it has one.
• The extended building must not be more than 7 m higher than the existing building.
• The roof of the extended building must not exceed 30 m in height.
• The floor to ceiling height of each new storey must not exceed 3 m, or the floor to ceiling height of the existing storeys, whichever is lower.

That last limit catches schemes designed to modern standards on a 1960s block. Existing floor to ceiling heights of 2.3 m or 2.4 m cap the new storeys at the same figure, which affects the quality of the flats you can deliver and the servicing zone available for the ceiling.

Works reasonably necessary to construct the additional storeys are also permitted, which includes engineering operations to strengthen existing walls and foundations, works for access including extending the lift and stair core, and provision of storage, waste and other ancillary facilities. That is a broad allowance and it covers most of what a real scheme needs.

The development must be completed within three years of the date prior approval is granted, which is tighter than it sounds once you add the building control route.

What the authority may consider

The authority is confined to the matters listed in paragraph A.2 of the Order, and the list is longer than for most prior approval classes:

Check the current text rather than this summary, because Part 20 has been amended since it was introduced and the list has moved.

Two of those matters do most of the work. External appearance, which in practice means whether the addition reads as part of the building or as a shed on the roof. And amenity, which means daylight and privacy for the people already living there.

The determination period is eight weeks, longer than the 56 days that applies to most prior approval classes, and the authority can refuse if the required information is not supplied. There is no deemed approval shortcut worth relying on here.

Daylight to the existing flats

This is the single most common reason a Part 20 application is refused, and it is entirely predictable.

Adding two storeys to a block puts a new mass directly above the top floor windows and, on an inner courtyard or a close elevation, above windows lower down. The existing residents lose light, and the Order makes the impact on their amenity a matter the authority must consider.

Commission a daylight and sunlight assessment to the Building Research Establishment guidance before the design is fixed, not afterwards. Report vertical sky component and no-sky line results for the affected rooms in the existing building, and annual probable sunlight hours where the elevation faces within 90 degrees of due south. Councils expect a suitably qualified author and professional review is strongly advised.

The new flats have their own test. The Order requires adequate natural light in all habitable rooms of the new dwellings, which means an internal daylight assessment as well as the impact study. Single aspect flats under a deep roof overhang rarely pass.

Where the results are marginal, the design responses that work are set-backs from the parapet on the affected elevation, reducing the footprint of the upper storey, moving the mass to the elevation facing a road rather than a courtyard, and specifying lighter-coloured surfaces to improve reflected light. Where the results are poor, the scheme needs a full application with a planning balance rather than a prior approval with a fixed list.

Overlooking is the other half of the amenity test. Rooftop terraces and balconies looking down into neighbouring gardens attract objection, and obscure glazing with restricted openings is the usual mitigation.

Structural capacity and the survey evidence

Part 20 requires a report on the structural strength and stability of the existing building, prepared by a chartered structural engineer or a chartered civil engineer, to accompany the application. That is a statutory requirement, not an optional extra, and an application without one is incomplete.

What the report has to establish:

1. The existing structural form, frame type and construction method, with reference to original drawings where they survive.
2. Foundation type, depth and bearing capacity, with trial pits where the records are absent.
3. Capacity to carry the additional load, including the lateral stability of the whole building under wind loading at the greater height.
4. Any large panel system construction issues, since blocks of that era may need appraisal against the disproportionate collapse requirements that followed the Ronan Point failure.
5. The strengthening works required, which are themselves permitted by the class, and their effect on the existing flats during construction.

Lightweight construction, whether cross-laminated timber, light gauge steel or modular units, usually makes the arithmetic work. It also brings its own fire and acoustic questions that building control will test.

A structural survey is the starting point for that engineer's report rather than a substitute for it, and councils expect a suitably qualified author. The question of which works are reasonably necessary also arises in other permitted development classes, as we discuss in Class Q structural works and what is reasonably necessary.

Fire safety and the gateway question

A Part 20 scheme often pushes a building over the higher-risk threshold, or adds storeys to a building already over it.

Where the completed building would be at least 18 m in height or at least seven storeys with two or more residential units, the Building Safety Act regime applies. That means building control approval from the Building Safety Regulator before any work starts, a completion certificate before occupation, and the second staircase requirement for new residential buildings above 18 m with its transitional arrangements.

Take advice early on whether a fire statement is required with the prior approval application for a relevant building, because the requirements have been amended and the position depends on the current text. It must be prepared by someone with the necessary fire-engineering competence in any event, and the fire strategy drives the core design.

Escape from the new storeys through the existing core is usually the hardest technical problem. The existing stair was designed for the building as built, and adding two floors changes travel distances, stair capacity and smoke control.

Leases, airspace and consents outside planning

None of this is planning, and all of it stops schemes.

The airspace above the roof has to be owned or demised to the developer. On a leasehold block the roof is often part of the common parts, and the leases may give leaseholders rights over it. Disposing of airspace can engage the right of first refusal under the Landlord and Tenant Act 1987, which means offering it to the qualifying tenants first and which carries criminal sanctions if ignored.

Check also for restrictive covenants, rights of light enjoyed by neighbouring owners as a private law matter separate from the planning daylight test, party wall notices, and the practical question of how you build above people's homes without a decant.

Resident engagement is worth doing properly even though it is not required. A block where the leaseholders are hostile produces objections on every prior approval matter, and the eight week period gives you little time to recover.

Preparing a prior approval submission that works

The pack that gets approved contains, as a minimum:

• Existing and proposed plans, elevations and sections with dimensioned heights and floor to ceiling dimensions.
• The chartered engineer's structural strength and stability report.
• A daylight, sunlight and overshadowing assessment covering both the existing building and the new flats.
• A short planning statement working through each paragraph A.2 matter under its own heading.
• Transport and refuse details, including where bins will be stored and collected.
• Contamination and flood screening, proportionate to the site.
• Contextual elevations and a material palette for the external appearance assessment.

And write the statement for the officer who has eight weeks and a list. One heading per matter, the evidence under it, and no general argument about housing need, which is not a matter they can consider.

This is general guidance, not legal advice; Part 20 has been amended repeatedly and a lawful development certificate is the only formal way to confirm that works are lawful, so check the current text with your local planning authority or a qualified professional before relying on it.

You can browse all 66 report types to see which documents a rooftop scheme usually needs. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.