Cat B fit out explained: shell and core, Cat A, Cat A plus and who pays for each

Fit-out categories are not defined in any Act or regulation. They are market shorthand, mostly from the office world, for how far a space has been built before a tenant arrives, and two buildings marketed as Cat A can be handed over in different states. The definition that counts is the one in the lease, the agreement for lease or the landlord's fit-out guide.

Shell and core: the building before anyone moves in

Shell and core is the developer's building with nothing inside the lettable floors. The frame, roof and facade are complete. The core is finished: stairs, lifts, risers, the toilets and the main plant. Water, power, ventilation and the sprinkler mains are brought up the risers and capped off at each floor, and the floor plate itself is bare slab with exposed soffits and no ceiling, no floor finish and no lighting beyond what is needed for safe access. In a parade of shops the equivalent is a retail shell: four walls, a slab, capped services and often no shop front.

Taking a floor in shell and core means paying for every layer that follows. It is the slowest and most expensive start, and the one with the most control, because the ceiling, air conditioning and lighting are laid out around the final plan.

What is a Cat A fit out?

A Cat A fit out is the landlord's base finish, applied across an open floor so it can be marketed and let. A typical Cat A specification includes a raised access floor or a finished screed, a suspended ceiling, general lighting, heating, cooling and fresh air distribution from the risers to terminal units on the floor, fire detection and alarm extended to cover the open area, sprinkler heads where the building has a sprinkler system, blinds to the windows, and finished toilets and common parts. What it does not include is anything that belongs to one particular business: no partitions, no meeting rooms, no kitchen, no reception desk and no branding.

Cat A services are laid out on an even grid for an open plan nobody has drawn yet, which is rarely right for the rooms a tenant then builds. That is why every Cat B package reworks some of them.

Cat A plus: a floor you can move into

Cat A plus, sometimes sold as fitted, plug and play or ready to occupy, is Cat A with a light tenant package added by the landlord before letting: a kitchen or tea point, one or two meeting rooms, floor finishes, furniture and often the data cabling and Wi-Fi. The landlord builds it speculatively to let smaller floors faster to tenants who do not want to run a project. The tenant moves in within days of signing, but the layout is someone else's guess, and the cost of the extra works is recovered through the rent or a service charge rather than paid up front.

What a Cat B fit out covers

Cat B is everything that makes the space belong to the occupier. It is designed by the tenant's own architect or interior designer, priced against their drawings and built after the landlord's Cat A is complete. On an office floor that usually means:

• Partitions, glazed screens, doors and the meeting rooms, offices and quiet rooms they form
• Reception, kitchen, breakout and tea points, with the plumbing and drainage to reach them
• Floor finishes over the raised floor, wall finishes, feature ceilings and acoustic treatment
• Lighting changes and controls to suit the new rooms, and any feature lighting
• Air conditioning and ventilation reworked so every enclosed room has supply, extract and its own control
• Fire alarm devices and sprinkler heads relocated and added to match the new layout, with the fire strategy updated
• Small power, floor boxes, data cabling, comms room, AV and access control
• Joinery, storage, signage and branding, and the furniture if it is in the contract

In a shop, café or clinic the same idea is called the tenant's works or the fit-out, and the list changes: a shop front and signage, a counter and display joinery, a kitchen with extraction and a grease trap, accessible toilets, clinical rooms with wash-hand basins and wipeable finishes, and the drainage and power each of those needs. Our pages on retail fit-out, café fit-out and clinic fit-out set out how those packages are built.

Cat A vs Cat B fit out, side by side

Who pays for what, and where rent-free periods come in

The usual split is simple: the landlord pays to bring the space to Cat A and the tenant pays for Cat B. What makes it negotiable is time. The tenant cannot trade while the fit-out is on site, so the heads of terms often give a rent-free period that covers the works and sometimes more, depending on the length of the lease and the state of the market. On a floor taken in shell and core, or where the tenant would rip out most of a new Cat A, the landlord may instead offer a capital contribution, cash in lieu of the Cat A works, and let the tenant build one package from slab to finished room.

Before agreeing either, have a contractor look at the space against the tenant's plan. The rent-free period should cover a realistic programme for design, approvals and the build, and a contribution should be weighed against the Cat A that would otherwise be removed.

Where Cat A meets Cat B, and how not to pay twice

The waste in most office fit-outs is at the join. A landlord completes a new Cat A ceiling, lighting and air conditioning grid, then the tenant's partitions cut through it: tiles are cut, fittings move, units are re-ducted and sprinkler heads relocated, at the tenant's cost. Agents call it Cat A wastage. If the tenant signs before the landlord's works start, agree which Cat A items are omitted. Get the as-built drawings before the Cat B design is fixed. And have one contractor survey both, so the price says which Cat A elements are kept, adapted or replaced.

Retail units have their own version: services capped at the rear, no shop front, or a slab not level enough for the chosen floor. Most centres and estates issue a tenant's fit-out guide listing what the landlord provides and the standards the tenant's works must meet. Read it before the design is priced.

Cat A and Cat B fit out in London: what changes on site

The categories are the same in London; the conditions are not. Multi-let buildings in the City, the West End and the large estates keep noisy work, deliveries and the goods lift to set hours, often evenings and weekends. Loading bays are booked by slot, there is rarely room for a skip, and waste leaves by a managed route. Older buildings in conservation areas or on the statutory list may need listed building consent before a partition touches original fabric. All of this belongs in the programme and the price before the contract is signed.

Approvals a Cat B fit out needs before it starts

Most leases require the landlord's written consent, a licence to alter, for anything beyond decoration, and the landlord's surveyor will want drawings, a method statement and the contractor's insurance first. Building control applies to most fit-out work, because new rooms change escape routes, alarm coverage and ventilation: the fire guidance is Approved Document B, and the fire risk assessment under the Regulatory Reform (Fire Safety) Order 2005 must be reviewed when the layout changes. Electrical, gas and extraction work is certified by the relevant specialist. A new shop front or external flue may also need planning permission.

Cat B fit out cost: what moves the number

No single rate per square foot means much for Cat B, because the scope is the tenant's design. The price rises with the number of enclosed rooms, each needing its own ventilation, lighting, alarm and sprinkler work; with glazed partitioning and acoustic specification; with kitchens, showers and new drainage; with the share of Cat A reworked or replaced; and with restricted working hours. Starting from shell and core adds the Cat A layer. Budget against a fixed price from a contractor who has surveyed the space and read the fit-out guide.

At the end of the lease: dilapidations and reinstatement

Most leases, and the licence to alter that permitted the works, require the tenant to remove its alterations at the end of the term, usually stripping the Cat B back to Cat A or to the condition recorded at the start. The landlord's surveyor serves a schedule of dilapidations listing the reinstatement, repair and redecoration claimed. A tenant who plans the strip-out early can often agree a cash settlement, or leave a fit-out the next tenant wants. Keep the handover photographs and the licence to alter with the lease.

Where a contractor like MO Team fits in

We build Cat B fit-outs for tenants and Cat A works for landlords on offices, shops, cafés and clinics across London, and the strip-out at lease end. We do not design: the tenant's architect or interior designer draws the scheme, a structural engineer handles anything structural, and we build to those drawings under a written fixed-price contract. Our own team does the main trades, and we act as principal contractor under CDM 2015 where the project needs one. On shop and café fit-outs we work out of hours and overnight where needed. See commercial fit-out, office fit-out and how we work.