A Cat B fit out is the second stage of building work on a commercial space, where the landlord’s base infrastructure remains untouched and the tenant completes all interior design, partitioning, mechanical systems, and finishes to their specification. It assumes the building shell and core services (structure, roof, external walls, risers) are already in place. Understanding the distinction between Cat A and Cat B work is essential when leasing a commercial property, as it determines your responsibilities, timeline, and budget allocation during the design and construction phases.
How does Cat B differ from Cat A work?
Cat A (Category A) and Cat B (Category B) are industry-standard terms that divide commercial fit-out responsibility between landlord and tenant. Cat A work is the landlord’s obligation: it covers the basic building envelope, structural elements, external fabric, and primary building systems including main risers for services (water, gas, electricity, drainage). The landlord delivers the space to Cat A standard, which is essentially a blank, weathertight shell with no internal divisions, no decorated surfaces, and no distributed services.
Cat B work begins where Cat A ends. The incoming tenant is responsible for all interior design, specification, and installation. This includes: all internal partitioning, all secondary mechanical and electrical distribution (branch runs from risers to individual spaces), decoration and surface finishes, door sets, glazing, flooring, ceilings, lighting design, HVAC terminals, and any bespoke or specialized systems the tenant requires. The tenant engages their own design team and contractors to deliver a fully finished, operational interior that meets their business needs.
What is typically included in Cat B work?
Cat B scope encompasses the entire interior environment that makes a commercial space functional for its occupant. Structural work includes all internal walls, stud partitioning, and structural alterations (within code and lease constraints). Mechanical, electrical, and plumbing (MEP) systems require the tenant to design and install all secondary distribution—that is, every pipe, cable, duct, and conduit that branches from the landlord’s primary risers into individual rooms, offices, and zones. This is a substantial undertaking, as it must integrate with building codes, fire regulations, and accessibility standards.
Interior finishes and fit-out items fall entirely to the tenant: flooring (whether timber, stone, carpet, or resin), wall surfaces (paint, paneling, tile, plaster), suspended ceilings or exposed finishes, all joinery and cabinetry, doors, frames, ironmongery, and glazing. Services termination—where heating, cooling, electrical outlets, and IT points end up in the actual space—is designed and specified by the tenant’s design team. Finishes and aesthetic choices reflect the tenant’s brand and operational requirements; these decisions are made during the Concept, Design & Specification stage of the project, where the studio works with the client to align interior choices with business identity and day-to-day function.
Why does the Cat A/Cat B split matter for your project timeline?
The Cat A/Cat B boundary directly affects how long your project takes from lease commencement to occupation. Cat A work is typically completed by the landlord before you take possession of the space; however, defects, delays, or disputes over what constitutes Cat A can extend this phase. Once you receive the Cat A shell, your Cat B work begins. This is where your design team undertakes Discovery to establish your exact needs, then moves into Concept, Design & Specification to develop layouts, materials, and systems. Each stage requires client sign-off and coordination with building control, planning (if required), and your contractors.
Commercial fit-outs rarely proceed in isolation. Parallel workstreams—MEP design, structural engineering, fire safety strategy, and specification of finishes—must be coordinated so that services don’t clash, buildability is confirmed, and procurement windows align with site start dates. Delays in any one discipline can push back occupation. Understanding that Cat B is your responsibility, not the landlord’s, means you control the timeline; it also means you must resource your design and project management from day one. The studio has managed this coordination across commercial projects including Beaulieu Dental Practice, Fruittii Hair Salon, Keystones Estate Agent, and Tone at Canary Wharf, where service routing, space planning, and material lead times all had to be locked in during Concept, Design & Specification to meet handover dates.
What design decisions are made during Cat B work?
During Discovery, the studio establishes what the space needs to do: how many people will work there, what equipment or machinery must be accommodated, what brand identity or aesthetic is required, and what regulatory standards apply (healthcare, food safety, accessibility). For Beaulieu Dental Practice, this meant understanding clinical workflows, infection control requirements, and patient experience. For Fruittii Hair Salon, it meant designing for water, drainage, electrical demand, and mirror placement that the Cat A shell could not provide.
The Concept, Design & Specification stage translates these needs into tangible drawings, material schedules, and engineering specifications. Interior walls are positioned to create the right number and size of spaces. Lighting is specified for both function (task lighting in a salon chair, general illumination in an office) and atmosphere. Flooring is chosen for durability, hygiene, and brand fit. MEP systems are routed from risers to every point of use—sink, cooker, workstation, server cabinet—with redundancy and future flexibility considered. Acoustic performance, if critical (quiet meeting rooms, sound dampening in a salon), is designed in. Colour, material finishes, and joinery express the tenant’s identity. None of this exists in Cat A; it is entirely the tenant’s choice and responsibility, and it is what transforms a bare shell into a working commercial space that reflects how the business operates and how it wants to be perceived.
Who is responsible for budget and risk in Cat B work?
Because Cat B is the tenant’s scope, the tenant bears the cost and the risk of delivering it. This includes design fees, specification development, contractor selection, procurement, installation, testing, and any remedial work if issues arise during construction. The landlord’s responsibility ends at Cat A handover; what happens next is between the tenant, their design team, and their contractors. This is why a clear brief at the start—and a thorough Discovery phase—is essential. Scope creep, late changes to specification, or discovery of unforeseen building conditions (structural surprises, service clashes, compliance issues) during construction can extend timelines and costs.
The studio mitigates this risk by front-loading the design process. During Discovery and Concept, Design & Specification, the studio gathers information, tests assumptions against the building fabric, and produces detailed specifications that leave no ambiguity for the contractor. By the time the Commission stage begins (when contractors are mobilized on site), the design is fixed, quantities are confirmed, and the build can proceed without constant rework. Transparency about what is known, what requires site survey confirmation, and what contingency might be needed protects the tenant from surprises.
How does Cat B fit-out affect lease terms and future flexibility?
Cat B work is permanent. Once partitions are built, services are installed, and finishes are applied, removing or altering them is costly and disruptive. For this reason, lease terms often specify what Cat B work the tenant must remove or make good before vacation (dilapidations). Some leases require the space to be returned to Cat A condition; others allow the tenant to leave Cat B work in place for the next occupant. Understanding these obligations before design begins is critical, as it shapes decisions about demountable partitioning versus fixed walls, proprietary versus standard systems, and material durability.
Flexibility within Cat B work is still possible but must be designed in from the start. Modular joinery, cable routes that allow rewiring, floor zones that permit service additions, and non-structural partitioning all preserve adaptability without compromise. The London Embankment Apartment and Witham Project demonstrate how careful specification of finishes and built elements can endure changing use while maintaining quality. If your business may grow, move, or evolve its spatial needs, this informs how the design team approaches layout and infrastructure during Concept, Design & Specification. Planning for change at the design stage is far simpler than retrofitting later.