Upward extensions under Part 20 of the GPDO

Part 20 of Schedule 2 to the General Permitted Development Order grants a right to add storeys to existing buildings, and almost every building an investor looks at fails one of its conditions. The right sits in five classes covering blocks of flats, commercial and mixed use buildings whether detached or in a terrace, and dwellinghouses whether in a terrace or detached. Each class carries a construction date window, an overall height ceiling, a limit on the height of the new storeys, a list of excluded land, and a prior approval application the local planning authority can refuse on specified grounds.

A part 20 upward extension is therefore permitted development with an application attached. The difference from a planning application is the scope of what the authority may consider, not the absence of a process.

Which buildings the classes cover

Class A applies to detached blocks of flats of three storeys or more above ground level, allowing up to two additional storeys of new flats. Classes AA and AB cover buildings in commercial or mixed use, detached and in a terrace respectively, allowing new dwellings above. Classes AC and AD cover dwellinghouses in a terrace and detached dwellinghouses, allowing the house itself to be enlarged upwards.

The distinction between enlarging a house and creating new dwellings matters. Under Classes AC and AD you are extending an existing dwelling, so the result is one larger house. Under Classes A, AA and AB you are creating separate dwellings, which brings in access, refuse, parking, amenity space and the management of a building in multiple occupation as planning considerations, and brings in leasehold and airspace ownership questions as legal ones.

Terrace has a defined meaning in the Order, and a building at the end of a terrace is treated differently from one in the middle. Read the definition rather than assuming, because semi-detached pairs and buildings attached only by a garage cause arguments.

Height limits and the date conditions

Three sets of numbers filter most schemes out:

• a construction date window. Each class requires the existing building to have been built within a defined period, beginning on 1 July 1948 and closing at a date in 2018 that differs between classes. Anything older or newer is outside the right entirely.
• an overall height ceiling. For additional storeys on blocks of flats the resulting building must not exceed 30 metres. For dwellinghouses under Classes AC and AD the ceiling is 18 metres.
• a limit on the new storeys themselves, expressed by reference to the floor to ceiling height of the existing storeys and capped at 3 metres internally, so you cannot add a generous storey on top of a building with low existing floor heights.

Establishing the construction date is the first piece of due diligence, and it is harder than it sounds on a building altered in the 1960s. Historic mapping, the local authority's building control records, the title and rating records are the usual evidence. Where the date is marginal, the honest answer is that the right is unavailable until you can prove otherwise, because the burden sits on the developer.

Verify each figure against the current consolidated text of the Order on legislation.gov.uk before you commit, since Part 20 has been amended more than once since it was inserted.

Matters for prior approval

The authority cannot refuse on general planning policy grounds. It can only consider the matters listed for the class, which typically include transport and highways impacts, air traffic and defence asset safeguarding, contamination risk, flood risk, the external appearance of the building, the provision of adequate natural light in all habitable rooms of the new dwellings, and the impact on the amenity of the existing building and of neighbouring premises, including overlooking, privacy and loss of light.

External appearance is the widest of those and the one authorities use most. It covers the design of the additional storeys and their relationship to the host building, which gives a case officer considerable room on a prominent elevation.

Natural light is the condition that defeats the most schemes. The requirement applies to all habitable rooms in the new dwellings, and a top-floor flat formed within a mansard with limited glazing frequently fails it. Model it properly rather than asserting it, and where the scheme is marginal the analysis belongs in a Daylight and Sunlight Assessment, which councils expect from a suitably qualified author.

The determination period is 56 days from a valid application. If the authority does not determine within that time, the development may proceed subject to the conditions in the Order, which is a different thing from a deemed approval on your terms.

Impact on amenity of neighbouring premises

This is where prior approval applications are most often refused, and the reasoning is usually about the relationship between the new storeys and windows on adjoining buildings.

Three analyses answer most objections. A daylight and sunlight study of affected neighbouring windows against the recognised methodology. A sections study showing the new massing against the neighbouring building, with the standard angular tests applied. And an overlooking analysis with window positions, sill heights and any screening or obscured glazing proposed.

The existing building's own occupiers count too. Adding two storeys above an occupied block affects the top floor flats, their outlook and their roof terraces where they have them, and an authority will weigh that. Tell those residents before the application is submitted rather than letting the site notice do it.

Construction management and engineering requirements

Part 20 requires a report with the application dealing with the construction, and the matters for approval include the construction phase. For Class A the Order expects a report dealing with the impacts of the construction, including hours of operation and transport and highways effects.

Treat it as a document rather than a paragraph. Set out the deliveries, the crane or hoist position, the scaffolding and any highway occupation needed, the hours, the dust and noise controls and the arrangements for keeping the existing building occupied and served throughout. A Construction Management Plan covering those points gives the case officer something to approve.

The engineering reality is harsher than the planning position. The existing frame and foundations have to take the load, which on a 1960s block often means a lightweight steel or timber frame, local strengthening, or a transfer structure. The stair and lift core must serve the new storeys, with means of escape for the whole building reassessed, and a building that crosses a height or storey threshold may fall into the higher-risk regime under the Building Safety Act 2022, with its own gateways and a different approvals programme. Check that position by reference to the finished building rather than to the planning route taken.

Excluded land and buildings

The right does not apply on Article 2(3) land, which covers conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites. It does not apply to a listed building or to land within its curtilage, to a scheduled monument, or on a site of special scientific interest, a safety hazard area or a military explosives storage area.

That list removes a large share of the stock that would otherwise be attractive, because the terraced commercial buildings with the best airspace value tend to sit in town centre conservation areas. Check the designations on the authority's policies map and on the national planning data service before you pay for a feasibility study. Other permitted development rights carry the same exclusions, and how the limits work for air source heat pumps shows the same pattern in a smaller context.

Where the rights are impractical in reality

Being honest about the hit rate saves money. Between the date window, the height ceilings, the excluded land, the natural light condition, the structural capacity of mid-century buildings and the leasehold position in the airspace above a block of flats, most candidate buildings fail at least one test.

Three further costs land after prior approval. The development is chargeable for the levy where a charging schedule is in force, and a notice of chargeable development has to be submitted before work begins, which people miss because there is no planning permission to remind them. The work must be completed within three years of prior approval. And the right does not override leases, covenants, rights of light at common law or the Party Wall etc. Act 1996.

A prior approval is also not a certificate of lawfulness. Where there is any doubt that the conditions are met, a lawful development certificate is the only formal way to confirm that the works are lawful, and it is worth the fee on a scheme where funding depends on the answer. Reform in this area continues, and the planning law timeline is where to follow it.

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. Enter your postcode on PlanScope to see which reports a prior approval application on your building is likely to need.