Adding a mezzanine inside an industrial unit

Most internal works are not development, so a mezzanine inside an industrial unit usually needs no planning permission at all. The exception is retail. If the unit is used for retail purposes and the mezzanine exceeds 200 square metres, it becomes development and you need permission before it is built.

That single threshold explains why a warehouse operator can install a storage deck over a weekend while a trade counter two units along has to make an application. The rest of the picture is parking, fire and building control, and those apply whether or not planning permission is needed.

When an internal floor counts as development

Section 55(2)(a) of the Town and Country Planning Act 1990 takes works affecting only the interior of a building, or not materially affecting its external appearance, outside the definition of development. A mezzanine deck bolted to the inside of a portal frame does neither of those things, so on the face of it no permission is required.

Section 55(2A), inserted by the Planning and Compulsory Purchase Act 2004, cuts across that. It allows a development order to specify circumstances in which internal works increasing the gross floorspace of a building do count as development. The order made under that power is aimed at retail, and it sets the threshold at 200 square metres of additional floorspace in a building used for retail purposes.

So the test has three parts: is the building used for retail, does the mezzanine exceed 200 square metres, and is the additional floorspace being created by the works. All three have to be satisfied before permission is needed on that ground alone.

The retail threshold and how it is measured

Measure the gross internal area of the new deck, including circulation and any plant enclosure on it, not the sales area. A 260 square metre deck of which 180 square metres is customer-facing is still over the threshold.

Cumulative works are the trap. Two decks of 130 square metres installed eighteen months apart in the same building will be read together where the second is plainly part of the same scheme, and a council can take enforcement action on that basis. Where the intention is to phase, say so in the first application.

Where the building has a mixed use, with a trade counter at the front and storage behind, the question is whether the building is used for retail purposes rather than whether the mezzanine sits over the retail part. That is a judgement on the facts, and it is one worth resolving with the council before installation rather than afterwards.

Parking standards keyed to floorspace

This is where a permission-free mezzanine still causes trouble.

Adopted parking standards are almost always expressed per square metre of floorspace, and a mezzanine adds floorspace. If the operator later applies for anything at all, a new access, an extension, a change of use, the council will assess parking against the total floorspace including the deck. An operator who added 400 square metres without permission can find the next application refused on parking grounds.

The same applies to servicing. More floorspace usually means more stock movements, and a yard that worked for the original unit may not swallow an extra delivery a day once turning and manoeuvring are drawn out properly.

Where the mezzanine does need permission, expect the highway authority to ask for the demand calculation. On a constrained site, a Parking Survey recording actual occupancy across representative days is a more persuasive answer than a standards-based figure, and a short Transport Statement is usually enough to carry it.

Means of escape and occupancy

A mezzanine adds people at a level with limited exits, and that drives the fire strategy.

The controlling questions are travel distances from the furthest point on the deck to a protected escape route, the number and width of stairs, whether the deck is open to the space below or enclosed, and the occupancy figure used in the calculation. An open deck of limited area serving storage only is treated very differently from an enclosed floor with staff workstations.

Sprinkler provision, smoke ventilation and the effect of the deck on the existing detection layout all follow from the same assessment. On larger or more complex installations a fire engineer prepares the strategy. Planning does not assess any of this, and a planning permission does not imply that the deck is compliant.

Whether the use class moves

Installing a deck does not change the use of the building. A B8 storage unit with a new picking mezzanine is still B8.

What changes the use is what happens on the deck. A storage unit that opens a showroom on the new floor has introduced a retail element, and depending on scale and prominence that may be a material change of use requiring permission, or it may be ancillary and require nothing. Class E allows movement between retail, professional services, indoor sport, medical and office uses without permission, but B8 and B2 sit outside Class E, and a move from B8 into any part of Class E is a change of use.

The practical consequence for a commercial occupier is that the mezzanine and the use should be considered together. Two separate questions, one decision.

Building Regulations and structural approval

Everything above concerns planning. Building control is a separate consent and the one that actually governs the installation.

A mezzanine is a material alteration under the Building Regulations 2010. The application covers structure, including the effect on the existing frame and slab, fire safety under Approved Document B, protection from falling under Approved Document K, and access under Approved Document M where the deck is used by the public or by staff in a way that requires it.

Two points that catch people out. The existing floor slab may not have been designed for point loads from mezzanine columns, so a slab investigation is often needed before the design is fixed. And a deck installed as "demountable storage equipment" to avoid the regulations is still a building element if it is fixed and occupied, whatever the supplier's brochure calls it.

What to submit for an internal alteration

Where permission is required, the application is smaller than most. Existing and proposed floor plans and sections at a recognised scale, a location plan, a site plan showing the yard and parking, the application form and the fee.

Supporting documents are proportionate. A short Planning Statement dealing with the floorspace increase, parking, servicing and the retail impact position where relevant. A Design and Access Statement is not usually required for an internal alteration outside a conservation area or a listed building, and offering one unasked adds nothing.

Where the additional floorspace is large and the unit is out of centre, the council may raise the retail sequential test and, above the national or local threshold, retail impact. That is a policy argument about trading floorspace rather than about the deck, and it needs answering with evidence about catchment and the nature of the goods sold.

One last practical note. Check whether a Community Infrastructure Levy charge applies before you build. Liability depends on whether permission is required and on the charging schedule, and charging schedules are indexed annually, so ask the charging authority for the current position rather than relying on a figure from a previous scheme.

Enter your postcode on PlanScope to see which reports a change to your unit 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.

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