Warehouse space that is paid for and not used is one of the more expensive things a business can own. So when volumes drop, or a lease was signed for growth that has not arrived yet, the obvious move is to let somebody else use part of it.

That is a commercial decision, and it is usually a sound one. What surprises people is how much of it turns out to be a layout and racking problem — because a warehouse is designed as one operation with one set of exits, one traffic pattern and one fire strategy, and splitting it in two changes all three.

Here is what actually has to change, in the order it matters.

First: is it permitted at all?

Before any drawing, three permissions have to be established, and none of them is ours to give.

Whether your lease permits subletting or sharing, and on what terms. Whether the landlord or managing agent consents, and what they will require in return — usually a say in who occupies the space and what they store. And whether your insurer needs to be told, because who else is in the building and what they keep there is material to a policy.

There is a fourth that people forget: whether the incoming party's activity is permitted in that building at all. A unit approved for one kind of use is not automatically available for another, and that question belongs to the landlord and the relevant agency, not to the two tenants shaking hands.

Settle these first. Everything below is wasted effort if the answer to any of them is no.

A racking run is not a wall

The instinct is to use the existing racking as the boundary — put a back-to-back run down the middle, and each side has its own space. It reads well on a plan and it is the cheapest thing to do.

It is worth being clear about what that boundary actually is. A racking run is a storage structure. It is not a fire separation, it is not a security barrier, and it does not stop a forklift. If the arrangement requires genuine separation — for fire compartmentation, for security, or because the two parties simply need to be able to lock their own space — that is a partition, and a partition is a building alteration with its own approvals rather than a racking exercise.

Where racking is used as a visual and operational divider only, it should still be treated as racking: anchored, load-noticed and inspected, and protected where forklifts from either side can reach it. Divider runs sit at the end of aisles on both sides, which is exactly where forklift impacts concentrate.

Escape routes are the part that gets missed

This is the one that turns a straightforward sublet into a redesign, and it is worth checking on the drawing before anything is agreed commercially.

A warehouse's exits were positioned for one operation occupying the whole floor. Split it, and each party now has access to some of those exits and not others. Anything that divides the space — racking, hoarding, mesh, a partition, stacked goods — changes how far somebody has to walk to reach an exit and which exits they can reach at all. Blocking a route that used to be available is not a small change; it is the kind of change that gets a building written up.

The practical test is simple and worth doing with a plan and a highlighter: from the furthest corner of each party's area, trace the route to the nearest usable exit, and check that the route does not pass through the other party's space or through a door they can lock. We went through the general principles in escape routes and exit access in a racked warehouse, and a sublet is the case where they most often stop working.

Sprinklers, height and anything you build

If the split involves building anything — a partition, a cage, a mezzanine to give one party its own floor area — the sprinkler design comes into it. Sprinkler coverage assumes an unobstructed space of a certain kind, and putting a deck or a solid partition under existing heads changes what is protected and what is now underneath something.

The same applies to stacking higher than before because you have less floor. Clearance between the top of stored goods and the sprinkler heads is a design requirement, not a preference, which is the whole subject of sprinkler clearance and racking approval. Losing floor area tempts people to gain it back vertically, and that is where it goes wrong.

Traffic: two fleets, one floor

The single biggest day-to-day friction in a shared warehouse is not storage. It is movement.

Two operations mean two forklift fleets with different drivers, different training and different urgency, and unless the layout separates them they will meet in the aisles. Decide explicitly whether traffic is shared or separated. If separated, each party needs its own aisle access to its own runs without crossing the other's working area — which usually constrains where the boundary can sit far more than the floor area split does.

Then there is the dock. One or two dock doors designed for a single operation's inbound and outbound rhythm now serve two, and the peak hours are frequently the same for both. Staging space is the thing that runs out first: the area in front of a dock door that was adequate is now being asked to hold two operations' goods, and part-pallets end up on the floor in the aisles. If the doors are shared, agree the time slots in writing before the first delivery, and read dock door alignment and rack runs for why the geometry in front of a door matters as much as the door.

Splitting an existing racking run

If the boundary falls in the middle of a run, you cannot simply take out the bays that are in the way.

A run of selective racking is braced along its length, and the end of a run is a different condition from the middle of one. Removing bays creates two new run-ends that need the frames, bracing and anchoring appropriate to a run-end. Removed bays also change the load path into the floor, and the beams and frames left behind may have been specified for a configuration that no longer exists.

The three things to sort out, and to write down:

• Load notices per party. Each run now belongs to somebody, and the safe working load displayed on it has to match what is actually installed after the change, not what it was before. A sign that is wrong is more dangerous than no sign, because people trust it. That is the point of load notices and SWL signage.
• Who inspects, and who reports damage. If the incoming party's forklift hits your racking, somebody has to see it, record it and act. Name the person, on both sides.
• Who owns the steel. It sounds obvious until the sublet ends and one party has added bays, moved beams or bought decking. Record what belonged to whom on day one, with photographs.

Floor loading is not automatically transferable

Your racking layout was designed around your stock. Another business's stock can be considerably denser — think tiles, batteries, drums, paper — and the same footprint can put far more load into the slab through the baseplates.

On a ground-floor slab in a purpose-built warehouse this is usually manageable. On an upper floor, on a ramp-up unit, or over a basement, it is a real constraint and it belongs to the building rather than to the racking. Ask what the incoming party will actually store and how heavy it is, before agreeing where they can put it, on the same basis as floor loading in ramp-up units.

Plan the exit while you are planning the entry

Sublets end. Either the other party leaves, or your volumes come back and you want your space.

Two things make that easy or painful. First, whether anything was drilled, built or altered, and who is responsible for putting it back — the same reinstatement question that governs your own tenancy, covered in racking in a leased warehouse. Second, whether the layout was designed so it can be reversed. A boundary that follows an existing aisle line can be moved in a weekend. A boundary that cut through the middle of runs cannot.

If there is any prospect of taking the space back, the cheapest decision you will make is putting the divide where the layout already wants it, even if that gives up a few per cent of area today.

Keep the records straight for both parties

A shared warehouse doubles the number of people who need to know what is installed and what it can carry, and halves the chance that anybody keeps it. Before the space is handed over, update the as-built layout, record the racking configuration and load notices on both sides, agree the inspection interval and who carries it out, and photograph the condition of everything on the boundary. What is worth keeping, and why, is in racking records: what to keep.

The short version

Settle the permissions first — lease, landlord, insurer, permitted use — because nothing else matters if one of them is no. Then treat it as a layout job: racking is not a wall, escape routes were designed for one occupier and have to be re-traced for two, anything built brings the sprinkler design into it, and traffic and dock staging cause more day-to-day friction than storage does. Splitting a run creates new run-ends that need to be built as run-ends, load notices must match what is actually installed, and floor loading does not transfer just because the footprint is the same. Put the boundary where the layout already wants it, and write down who owns which steel before anyone moves in.

If you are considering giving up part of your space, or taking part of somebody else's, send us the unit and how you are thinking of dividing it. Message us on WhatsApp, or request a site survey and we will walk the floor and tell you where the line can sensibly go — and where it cannot.