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Paragraph C: Part 20 definitions
Paragraph C supplies the meanings used across Part 20’s rights for new dwellings above existing buildings and for demolition and replacement under Class ZA. A wrong reading of “storey”, “detached” or “row” can change whether a proposal qualifies.
The definitions below summarise Paragraph C of the current 2015 GPDO. Read them alongside the limits and conditions of the specific Part 20 class. This guidance covers England.
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Defined building and site terms
| Term | Meaning for Part 20 | Practical point |
|---|---|---|
| Detached | A building without a party wall shared with a neighbouring building | Check actual party walls when choosing a detached or terrace class |
| Terrace building | A building that is not detached | The statutory definition includes a semi-detached building, subject to the class’s other tests |
| Row | Two or more terrace buildings joined by party walls or with main walls adjoining, with specific rules for the end buildings | A row can contain just two buildings. The height tests for Classes AB and AC refer to every other building in the row |
| Principal part | The main building excluding any lower front, side or rear extension, whether original or later | Measure class-specific rooftop limits against the relevant principal part where the class directs |
| Purpose-built | Built as, and remaining, a block of flats or dwellinghouse, as applicable | A later conversion is not automatically a purpose-built block for Class A or ZA |
| Block of flats | A horizontally divided building with separate self-contained dwelling premises, plus facilities built solely for its occupiers | The Part 20 definition is specific to these rights |
| Flat | Separate, self-contained premises constructed for use as a dwellinghouse | Assess the class’s stated building and use requirements |
| Footprint | The total ground area covered by a building | Relevant to Class ZA’s replacement footprint limit |
| Habitable rooms | Rooms used or intended for sleeping or living, excluding rooms solely for cooking, bathrooms, toilets, services, corridors, laundry, halls and utility rooms | Natural light prior approval is assessed for every habitable room in the new homes |
| Agricultural tenancy | A tenancy under the Agricultural Holdings Act 1986 or Agricultural Tenancies Act 1995 | Relevant to Class ZA’s agricultural tenancy restriction and consents |
| Defence asset | A site on a safeguarding map supplied to the authority under the relevant procedural direction | Triggers the relevant air traffic and defence assessment and consultation |
| Technical sites | The meaning in the Safeguarded Aerodromes, Technical Sites and Military Explosives Storage Areas Direction 2002 | Relevant to safeguarding advice and consultation |
How Part 20 counts a storey
Paragraph C(2) excludes a storey below ground level and accommodation within a roof, whether original or added later. If a Part 20 additional storey replaces existing roof accommodation, it still counts as an additional storey. Do not treat an occupied attic as an existing storey to gain an extra rooftop storey under a class.
The article 9A fire safety height test has its own measurement rules. Its 18 metre height is measured from ground level to the top floor surface of the top storey, disregarding plant-only top storeys. For its seven storey alternative, below-ground storeys are ignored and a mezzanine with at least half the floor area of the largest above-ground storey counts. The Article 9A test must be applied when Paragraph C(3) points to it.
The fire risk condition added to Paragraph C
Paragraph C(3) says a Part 20 development meets the fire risk condition where the building after the development will contain at least two dwellinghouses and meet the article 9A height condition: 18 metres or more, or seven or more storeys. Only Classes A and AA make fire safety impacts a prior approval matter when that condition is met, under A.2(1)(j) and AA.2(1)(l). For those applications, Paragraph B(2)(h)(ii) and B(6A) require a fire safety design statement and consultation with the Health and Safety Executive. Classes ZA, AB, AC and AD have no equivalent fire safety impacts prior approval matter in Part 20.
This fire risk trigger is different from the separate 18 metre existing building external wall matter and professional report for Classes A and AA under Paragraphs A.2(1)(i), AA.2(1)(k), B(2)(i) and B(9A). Check which test applies to the proposal.
Other requirements outside Paragraph C
The class pages set eligibility, height, use and timing limits. Article 3(9A) separately requires each new dwellinghouse to have at least 37 square metres gross internal area and meet the nationally described space standard. Article 4 also allows directions withdrawing specified permitted development rights, subject to its provisions. Neither rule is replaced by these definitions.
Primary legislation
Explore GPDO Part 20
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Page updated: 24th September 2026