Part 20: Construction of New Dwellinghouses in Airspace

Part 20 of Schedule 2 to the General Permitted Development Order (GPDO) grants tightly defined rights to create new homes above certain existing buildings in England. Class ZA also covers the demolition and replacement of one qualifying detached building. These rights are subject to detailed limits and, crucially, a prior approval procedure. A proposal is not automatically permitted merely because there is room above a roof.

The six classes distinguish a purpose-built block of flats, commercial or mixed-use buildings, and existing houses. The building’s present and historic use, age, height, position in a row and site constraints determine the route. The old A1, A2, A3 and B1 descriptions are references in the legislation; they should not simply be read as all present-day Class E uses.

This guide covers England. For the operative tests, read the current text of Part 20 alongside the relevant class page.

Choose the right Part 20 class

Each class has its own eligibility dates, storey and height limits, design restrictions and conditions. Compare the six classes for a quick starting point; the class pages give the detailed tests.

Checks before making a Part 20 application

• Building and use: establish the lawful current use, the specified use on 5th March 2018 where relevant, construction dates and any prior upward extension. Class ZA has different demolition, vacancy, footprint and date tests
• Location: check the class-specific exclusions, including article 2(3) land such as conservation areas, listed buildings and their curtilages, scheduled monuments, sites of special scientific interest, safety hazard or military explosives areas and land within 3 kilometres of an aerodrome
• Local restriction: check whether an Article 4 Direction has removed the relevant right at the site. Article 4 can apply to Part 20, subject to the order’s transitional rules
• Dimensions and design: calculate the permitted new storeys and overall height, as well as any terrace roofline, footprint, engineering, façade and access restrictions in the chosen class
• Separate consents: permitted development rights do not settle building regulations, property ownership, lease or party wall issues, or other statutory controls

Prior approval and completion

A qualifying proposal must follow Paragraph B’s prior approval procedure. The council considers the specified matters for the class, including matters such as neighbouring amenity, design, natural light, transport and highways, flooding and contamination. Some proposals have further heritage, aviation or fire-safety scrutiny. Part 20 requires the authority’s written prior approval before development begins; do not assume that silence after 56 days is approval.

The new dwellings must comply with article 3(9A)’s minimum 37 square metres gross internal floor area and the nationally described space standard. Each class also restricts the use of the resulting homes to Class C3 dwellinghouses and imposes a three-year completion condition with completion notification. Read the relevant class and Paragraph C’s definitions for the exact statutory wording.

Part 20 is a planning permission route for particular buildings and works. If any eligibility test or condition fails, a planning application may be needed.

Explore GPDO Part 20

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Page updated: 24th September 2026