The answer depends on the existing lawful building, the proposed development and the relevant national and local policy tests; calling land “grey belt” does not itself give permission. [1] [2]
Check the designation and the planning route
Start with the council's adopted policies map and the property's planning history. In Epping Forest District, use the district's adopted Local Plan, adopted on 6 March 2023, alongside national policy. For London sites, check whether the land is Green Belt or Metropolitan Open Land: these are distinct designations, not alternative names for any green space. [3] [2]
A planning-policy assessment is different from a permitted development assessment. Check any householder rights against the national rules and property-specific restrictions rather than assuming the designation settles whether an application is needed. [4] [5]
Extensions and replacement buildings
The NPPF published on 17 August 2026 sets out the current Green Belt tests in policies GB6–GB8. GB7 includes a route for reusing, extending, altering or replacing an existing lawful building of permanent and substantial construction. An extension or alteration must not produce a disproportionate increase in size compared with the original building; a replacement must be for the same use and not materially larger than the building it replaces. [6] [1]
For the extension test, “original” means the building as it stood on 1 July 1948 or, if built later, as originally constructed. Earlier extensions therefore matter. Provide drawings or other reliable evidence identifying the original building, later additions and the proposal instead of measuring only the extra space now requested. [1]
There is no single percentage allowance stated in that national policy. Check the council's relevant policy and assessment approach; do not substitute an estate agent's suggested percentage for the actual test. Satisfying the Green Belt category also does not settle separate design, heritage or flood-risk issues. [1] [7] [8]
New homes, grey belt and very special circumstances
GB7 contains several other categories, including limited village infilling and certain redevelopment of previously developed land. Its grey-belt route has additional requirements concerning unmet need, sustainable location and the remaining Green Belt's purposes. Major housing development also has relevant “Golden Rules” concerning contributions. These are conditional policy routes, not permission for every unused plot. [1]
Development outside the GB7 categories is inappropriate. Under GB6 it should not be approved unless very special circumstances clearly outweigh the Green Belt harm and any other harm. That is a different test from simply showing that a design would be attractive or useful to its owner. [1]
Metropolitan Open Land in London
Policy G3 of the London Plan 2021 gives Metropolitan Open Land the same status and level of protection as Green Belt and applies national Green Belt tests for inappropriate development. Establish the site's mapped boundary and the relevant borough policies before preparing a proposal. [2]
The draft London Plan published on 16 July 2026 proposes revised approaches through PV6 for Metropolitan Open Land and PV7 for Green Belt. As checked on 2 October 2026, these remain draft policies, not an adopted replacement for G3. A proposal depending on their future wording needs to be identified as such, rather than presented as already permitted. [9] [10]
For a feasibility brief, collect the designation map, original-building evidence, existing permissions and an outline of the intended use and size. Assess the relevant policy route before paying to develop a detailed scheme on an assumed allowance.