Industrial and warehouse extensions under Part 7

Industrial extension permitted development sits in Schedule 2 Part 7 of the Town and Country Planning (General Permitted Development) (England) Order 2015. Class H allows the extension or alteration of an industrial building or warehouse, Class I allows hard surfaces within the curtilage, and a further class deals with plant and machinery. Used properly these rights let an occupier add production or storage space without an application, which on a lease with a break clause coming up can be the difference between staying and moving.

Used carelessly they produce unauthorised development, because the limits are tighter than they look and the conditions attached to Class H catch people who read only the floorspace cap. Read the current text of Part 7 on legislation.gov.uk before relying on any figure, because the limits have been amended more than once and they are tighter on article 2(3) land, which includes conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites.

The floorspace and cubic content limits

The Class H cap is built in a way that trips up anyone who takes the headline figure alone. It is expressed as the lesser of an absolute floorspace figure and a proportion of the gross floor space of the original building. On a small unit the percentage bites first; on a large one the absolute figure does.

Two words in that sentence do the damage.

"Original" means the building as it stood at the base date used by the Order, or as built if later, not as it stands today. Every previous extension counts against the allowance, including extensions carried out by a previous occupier and extensions granted planning permission. An industrial unit on a 1970s estate has usually been extended at least once, and the remaining allowance may be nil.

"Gross floor space" means all floors, measured internally. A mezzanine installed for storage counts, which surprises occupiers who treated it as racking. If a mezzanine was installed without an application, its floor area may also be unauthorised development in its own right, depending on its scale and function.

So the first task is documentary rather than technical: establish the original building's footprint and floor area from the planning history, then subtract everything added since.

Height limits and the boundary distance condition

Height under Class H is controlled by reference to the existing building rather than by an absolute figure, so an extension cannot rise above the building it extends. For a low eaves 1980s shed that constrains the clear internal height available, which is often the whole point of the extension.

The boundary condition is the one that defeats the most schemes. Class H excludes development within a stated distance of the site boundary where the land adjoining that boundary is in residential use, and the same approach applies to land used as a school. On a tight industrial plot backing onto housing, the usable area for a permitted extension can be a strip too narrow to build in.

Measure it properly. The distance runs from the boundary, not from the neighbour's building, and the relevant question is the use of the adjoining land rather than the use of the land across the road. Where the adjoining land is in mixed use, or is a residential garden belonging to a commercial building, get advice rather than assuming.

The restriction on introducing new industrial processes

This is the condition most often breached, and it is breached in good faith. Class H permits the extension to be used for the industrial process or the storage and distribution use already carried on at the site. It does not permit a new industrial process to be introduced in the extension.

The practical consequence is blunt. An occupier who extends under Class H and then installs a spray booth, a foundry operation, a food process or anything producing emissions, noise or waste of a different character from the existing activity has carried out development requiring permission, whatever the floorspace position.

Where the business is changing what it does, that change is the planning question and the extension is secondary. Apply for permission for both together and deal with the environmental effects properly in the application. A new process near a residential boundary will attract a request for a Noise Impact Assessment, and councils normally expect a suitably qualified author for acoustic work, with professional review strongly advised.

Hard surfaces and plant within the curtilage

Class I permits hard surfaces within the curtilage of an industrial building or warehouse, provided they are used for the purposes of the undertaking. On article 2(3) land the surface has to be permeable or drain to a permeable area within the curtilage, which is a drainage design question rather than a paperwork one.

Even off designated land, surface water is the issue to watch. Converting 600 m² of gravel yard to impermeable concrete changes the run-off regime, and where the site drains to a sewer the undertaker may have a view. On a site with any surface water history, a short Drainage Strategy is a sensible precaution even where no application is needed.

Plant and machinery within the curtilage has its own class with its own height limit, generally tied to the height of existing plant or to an absolute figure, whichever is lower. Dust extraction, chillers and silos are the usual candidates, and they are also the usual source of complaints.

Loss of parking and the servicing test

Nothing in Part 7 says the extension must leave adequate parking or servicing. That silence is a trap rather than a permission.

Two things can still bite. Conditions on the original planning permission for the estate or the unit frequently require parking and manoeuvring areas to be retained and kept available. Article 3(4) of the Order means a condition like that overrides the permitted development right, so building over the service yard is a breach of condition even though Class H would otherwise allow the extension.

And where an extension leaves no room for an articulated vehicle to turn, the practical result is reversing onto the estate road or the public highway. That is an enforcement risk under other regimes and a commercial problem whatever the planning position. Run the swept paths before committing, and if an application does become necessary, a Transport Statement with the swept path drawings answers the highway officer's first question.

An illustrative example

Illustrative only: a 1,900 m² warehouse on a 1980s estate, occupied by a distribution business wanting 400 m² more storage. The planning history shows the original 1984 building was 1,500 m², with a 400 m² extension approved in 1998. The remaining Class H allowance is therefore calculated against the 1,500 m² original, not the 1,900 m² as built. The proposed extension would sit 8 metres from the rear boundary, beyond which is a residential garden, so the boundary condition excludes it. The occupier applies for planning permission instead, submitting a transport statement with swept paths and a noise note on the additional dock doors. One extension, no permitted development route, and three weeks saved by checking the history first.

Sites near residential boundaries

Where housing adjoins, assume an application rather than permitted development and plan for the issues an officer will raise: noise from the extension's plant and from vehicle movements, light spill from new external lighting, hours of operation, and the visual effect of a larger building on gardens.

The measures that work are unglamorous. Position dock doors away from the shared boundary. Specify acoustic louvres on plant from the start rather than retro-fitting them after a complaint. Use flat glass luminaires aimed into the site. And offer a condition on hours if the business can live with it, because an offered condition is worth more than a resisted one.

Conditions and the record of the original building

Keep a file. The documents that matter are the original planning permission and approved drawings, every subsequent permission with its conditions, measured floor areas for each phase, and photographs of what exists now.

Two reasons. First, the Class H allowance cannot be calculated without them, and a purchaser's solicitor will ask. Second, where permitted development has been relied on, there is no decision notice to prove it, so your file is the only record that the works were lawful.

If certainty is needed, a Lawful Development Certificate under section 191 or 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that works are lawful or would be lawful. An email from a planning officer is helpful and not binding. Where an application turns out to be necessary, a short Planning Statement setting out the planning history and the floorspace arithmetic saves the officer a day and tends to be repaid in goodwill. Enter your postcode on PlanScope to see which reports an industrial extension at your site is likely to need.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.