A house-to-flat conversion normally needs planning permission, and borough policies can impose different tests for the size of the house and the retention of family accommodation. Brent, Waltham Forest and Merton illustrate why one borough's threshold should not be used to assess a property elsewhere. Building regulations are a separate approval question. [1][2][3][4]
Brent: the policy allows an acceptable extension to count
Brent's adopted Local Plan policy BH11 concerns converting a family home with three or more bedrooms into two or more dwellings. Its ordinary criteria require the existing home to be 130 m² or larger, or capable of an acceptable extension to that size, at least one resulting three-bedroom dwelling, and a location with PTAL 3 or above. PTAL is the public-transport accessibility measure used in the policy. [2][5]
The family dwelling should preferably have direct access to a garden or amenity space. The policy also identifies a narrow exception where the existing home's amenity is so deficient that family occupation is unlikely and the deficiencies could not reasonably be overcome. It is not an unrestricted exemption whenever the existing garden is small. [2]
The important comparison point is that Brent's wording expressly considers acceptable extension potential. Calling its threshold an “original house must already be 130 m²” rule would misstate the published policy. [2]
Waltham Forest: original floorspace is expressly important
Waltham Forest's Policy 20 uses gross original internal floorspace. It resists the specified conversions below 124 m², while its positive conversion criteria refer to homes with more than 124 m². Do not silently rewrite that wording as a clear entitlement at exactly 124 m². [3]
The policy requires a larger family home to be retained within the proposal, compliance with dwelling space standards, appropriate amenity space, cycle and refuse provision, and PTAL 3 or above. It also addresses additional on-site parking. Passing the initial floorspace test is therefore only part of the assessment. [3]
Unlike Brent's wording, this threshold explicitly refers to original floorspace. Keep original area and later additions separately identified in the survey evidence rather than assuming the same extension allowance applies in both boroughs. [3][2]
Merton: retain family accommodation where it is lost
Merton's adopted housing policy H11.1 requires re-provision of at least one family-sized home where an existing family-sized home would be lost. The supporting text identifies family-sized homes as having three or more bedrooms. Its housing chapter also addresses mix, design and accessibility. [4][6]
This is a different policy formulation from copying Brent's 130 m² test. The cited Merton provision is a family-housing replacement requirement; it should not be presented as a London-wide minimum-house-size rule. Nor does the absence of a figure in that particular provision establish that every other local design requirement has been met. [4]
Separate the measurements and the layout
A practical comparison schedule should identify the existing use and bedroom count, original and current internal areas, any proposed extension, the resulting dwelling mix and the accessibility evidence required by the applicable policy. These distinctions follow directly from the different borough tests. [2][3][4]
Then test the proposed homes themselves. A building large enough to enter the policy assessment is not automatically capable of producing the required family home, smaller flats, circulation and outdoor provision to the applicable standards. Waltham Forest's explicit additional criteria demonstrate why a single total-area calculation is not enough. [3]
Apply the current national context as well
The NPPF published on 17 August 2026 requires the weight of materially inconsistent local provisions to be considered under Annex A. It does not make every older conversion policy irrelevant solely because of its adoption date. Any claimed conflict should be identified precisely, rather than assuming that general support for housing automatically removes all family-housing safeguards. [7][8]
The draft London Plan's housing-mix proposals remain separate from adopted requirements. Record them as emerging policy and re-check their status when the application is assessed. [9][10]