23/10/2026
When Planning Permission is NOT Needed
(Mobile / Caravan Route)
You can often bypass full planning permission if the portable annex meets these core criteria:
• Moveable structure: It must be built as a caravan or mobile home (complying with standards like BS3632) rather than a permanent masonry or concrete-foundation fixture. It is constructed in sections and can be lifted or towed away.
• Size limits: It must generally stay within the legal caravan dimensions (typically up to 20m x 6.8m) and have a maximum internal height of 3 meters (or up to 3.05m depending on specific Caravan Act interpretations).
• Ancillary use: It must be used by a family member or dependent (e.g., a granny annex) and remain tied to the main house—sharing the same household, garden, and general utilities rather than functioning as a completely separate rented or sold home.
• Location: It must sit strictly within the cartilage (boundary) of your residential property.
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When Planning Permission IS Needed
You will need to apply for planning permission (or it will be contested) if:
• Permanent foundation: It is built with permanent structural foundations or traditional brick/block building methods rather than a movable chassis.
• Separate dwelling: It is completely independent—meant to be rented out to a stranger, sold off separately, or equipped with its own distinct postal address and separate utility billing.
• Fails Permitted Development (Class E): If it is treated as a standard brick/timber garden outbuilding with living/sleeping quarters, it loses standard Class E permitted development rights (which only cover incidental uses like a gym or home office, not sleeping/cooking accommodation).
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A Portable Building moved by a HIAB (hydraulic crane truck) can come under the Caravan Act (specifically the Caravan Sites and Control of Development Act 1960 and the Caravan Sites Act 1968), only if it satisfies strict statutory tests regarding size, construction, and mobility.
Being lifted or transported by a HIAB shows it is physically capable of being moved, which fulfills the mobility requirement, but it is not automatically classified as a caravan just because a crane can lift it.
Core Legal Requirements to Qualify
To legally count as a caravan under UK planning law, the portable building must meet all of the following criteria:
• 1 - Size Limits:
o Maximum length: 20 meters (excluding any drawbar)
o Maximum width: 6.8 meters
o Maximum internal height (floor to ceiling): 3.05 meters
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• 2 - Construction & Assembly:
o Must be designed or adapted for human habitation (if used as a dwelling, granny annex, or living space rather than purely industrial/business storage).
o If it is a twin-unit structure, it must be constructed in no more than two separate sections that can be transported and bolted or clamped together on-site.
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• 3 - Mobility:
o Once assembled, it must remain physically capable of being moved as a single unit or in two parts by road (via lorry or crane). It does not need wheels, or a towbar, or a permanent chassis, as long as it is not permanently fixed to a permanent foundation (like a concrete slab that turns it into operational development).
Recommended Next Steps
• Apply for a Lawful Development Certificate (LDC): Even if you believe your portable annex is exempt under the Caravan Act, you can apply to your Local Planning Authority
• for a Certificate of Lawfulness to get written proof that planning permission is not required.
• Check local rules: Conservation areas, listed buildings, or specific local council restrictions can override general mobile home exemptions.
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