Cat A vs Cat B fit-out: what changes in the HVAC

Cat A and Cat B are the two stages of fitting out a UK office. Cat A (Category A) is the landlord's base build: a lettable floor with working services. Cat B (Category B) is the tenant's own fit-out, which turns that floor into their workplace. For the heating, ventilation and air-conditioning, the boundary between them decides who designs, pays for and maintains each part of the system. It is also where many of the surprises in a fit-out programme come from.

This guide explains what each stage includes for the HVAC, what typically changes between them, and the questions worth asking before the layout is fixed.

1What is a Cat A fit-out?

A Cat A fit-out delivers a floor that is ready to occupy but not yet arranged for any particular tenant. The exact specification varies by building and landlord, but a Cat A office floor usually includes:

• raised access floors and a suspended ceiling,
• lighting to an open-plan layout,
• finished toilets, core and lift lobbies,
• base-build fire detection and alarm,
• heating, cooling and ventilation designed for an open-plan floor.

Cat A is a landlord's product. It is designed to suit as many future tenants as possible, so the services are laid out evenly across the floor for an assumed number of people.

What the HVAC includes in Cat A

A typical Cat A office installation in the UK has:

• Fresh air from a central air-handling unit or heat-recovery unit, usually on the roof or in plant space, brought down a riser in the core and ducted across each floor.
• Terminal units for heating and cooling, most often ceiling-void fan-coil units (water or refrigerant) or VRF ceiling cassettes, set out on a regular grid.
• Supply diffusers in the ceiling, fed from each unit by short flexible ducts, and return grilles taking room air back to the units.
• Extract from the toilets and core, with general extract or a return path for the office air.
• Controls zoned for open plan, typically one zone per group of units by each façade and in the middle of the floor.

All of it is sized for an assumed occupancy density, often somewhere between one person per 8 m² and one per 10 m², and for an even spread of heat from people, lighting and equipment.

2What is a Cat B fit-out?

A Cat B fit-out is everything the tenant adds to make the space their own: partitions and meeting rooms, kitchens and tea points, breakout and collaboration areas, comms rooms, reception, finishes, furniture and branding. It also covers every change to the building services those spaces need.

For the HVAC that last part is easy to underestimate. A Cat A grid of fan-coil units works well under an open ceiling. Put partitions under it and the grid no longer matches the rooms.

3What changes in the HVAC during a Cat B fit-out

The typical Cat B mechanical changes are:

1. Relocating terminal units. A unit that ends up straddling a new partition, or sitting in a small focus room it would overcool, is moved. Moving a unit means moving its pipework, condensate drain, fresh-air branch, supply ducts and controller.
2. More fresh air to dense rooms. A boardroom for sixteen may stand where the Cat A design assumed two or three desks. It needs fresh air sized for the room, often through a new branch with its own damper. That damper is frequently controlled by a CO₂ sensor, so the extra air is only delivered when the room is in use.
3. New extract. Tea points, kitchens, print rooms and showers need extract, sometimes through the façade and sometimes back to the core.
4. Comms and server room cooling. These rooms run all year round, so they usually get a separate split or VRF unit that doesn't depend on the office system's operating hours.
5. A way for air to leave closed rooms. Transfer grilles over doors, or ducted returns, let air out of meeting rooms, so they aren't pressurised and the fresh air actually flows through. Acoustic transfer units stop conversations carrying.
6. Acoustic treatment. Cross-talk attenuators on shared ductwork stop sound travelling from one room to the next through the ceiling void.
7. New controls zoning. Each enclosed room typically needs its own temperature control, so a meeting room can be cooled without chilling the open plan next to it.

On a drawing, many of these changes look small. A relocated fan-coil unit is a rectangle that has moved a few metres. In the ceiling void it means new refrigerant or water pipework, a new condensate route, re-routed flexible ducts, a new fresh-air branch and new wiring.

4Why the Cat A layout stops working when you add rooms

The underlying reason is simple: the Cat A design spreads fresh air and cooling evenly, and people don't sit evenly.

Take a single fan-coil unit with 75 l/s of fresh air ducted to it. Under an open ceiling that serves six or seven desks at a comfortable 10–12 l/s per person. Build a boardroom around that unit and put sixteen people in it for an afternoon, and the same 75 l/s is shared between sixteen. That is under 5 l/s per person, well below the regulatory minimum for an office. Carbon dioxide in the room climbs steadily towards the levels where people start to notice stuffiness and lose concentration.

The picture under this heading shows it on a fictional floor. With the Cat B walls built on top of the Cat A services, a full boardroom heads past 1,500 ppm of CO₂. With a Cat B design that adds a dedicated fresh-air branch, it stays below 900 ppm. There is more on the numbers in how much fresh air an office needs per person.

5Cat A+ and "plug and play" fit-outs

Many landlords now offer Cat A+, sometimes called plug-and-play or fitted space. A Cat A+ floor includes some Cat B elements, typically meeting rooms, a kitchen, furniture and data cabling, so a tenant can move in quickly.

For the HVAC, Cat A+ means the landlord's designer has already resolved a layout with rooms in it. That can be a real advantage. It also means a tenant who wants a different layout may be paying to change a fit-out that already exists. When you compare a Cat A+ floor with a Cat A floor, ask for the mechanical layout and the design occupancy of each room, not only the floor plan.

6Who pays for what

There is no single rule. Responsibility is set by the lease and by any landlord's contribution towards the fit-out. In general terms:

• The landlord pays for the Cat A installation and owns the base-build plant, risers and primary distribution.
• The tenant pays for the Cat B changes, including alterations to landlord equipment that the Cat B requires, and usually needs the landlord's consent for them.
• At the end of the lease, a tenant may have to reinstate the Cat A layout. That can mean moving fan-coil units back and removing new branches, so it is worth designing Cat B changes to be reversible where possible.

7Questions to ask before the Cat B layout is fixed

• What occupancy density was the Cat A system designed for, and how does the planned headcount compare?
• Which rooms will be busiest, and how much fresh air will each of them receive per person?
• Which terminal units are reused, relocated or new, and what goes with each move (pipework, condensate, controls)?
• How does air leave each closed room?
• Which rooms need cooling outside normal hours, and how is it provided?
• Is there spare capacity in the landlord's riser and plant for the extra fresh air?
• What has to be reinstated at the end of the lease?

These questions are easier to answer, and to agree, when everyone is looking at the same picture. Our guide to presenting an HVAC scheme to clients covers how to make that picture, and age of air explained shows how to check that fresh air actually reaches each new room.

8Frequently asked questions

What is the difference between Cat A and Cat B fit-out?

Cat A is the landlord's base-build fit-out: a lettable floor with raised floors, ceilings, lighting, toilets, and heating, cooling and ventilation laid out for open plan. Cat B is the tenant's fit-out on top: partitions, meeting rooms, kitchens, finishes, furniture, and the changes to building services those spaces need.

Does a Cat B fit-out change the air-conditioning?

Usually, yes. Units that end up in new rooms are moved or added, dense rooms such as boardrooms get extra fresh air, tea points and kitchens get extract, comms rooms get their own cooling, and controls are re-zoned so each enclosed room can be controlled on its own.

What is a Cat A+ fit-out?

Cat A+ is a Cat A floor with some Cat B elements already installed, typically meeting rooms, a kitchen, furniture and cabling, so a tenant can move in with little work. Its HVAC is designed for that specific layout rather than for open plan.

Why do meeting rooms get stuffy after a fit-out?

Because the Cat A fresh air was sized for an even spread of desks. A meeting room built around one unit's share of fresh air can hold far more people than that share was meant for, so fresh air per person falls and CO₂ rises. A Cat B design fixes this with dedicated fresh air to the dense rooms.

Who pays to change the HVAC in a Cat B fit-out?

Normally the tenant, as part of the Cat B works, with the landlord's consent for any changes to base-build equipment. The lease sets the detail, including whether the Cat A layout has to be reinstated at the end of the term.