If you’re diving into the world of commercial construction in the UK, you’ve probably heard the terms Cat A and Cat B thrown around. They’re key stages in fitting out a building, especially for offices, and they play a big role in turning a raw space into a functional workplace. While they’re both about getting a building ready for tenants, they serve very different purposes. Let’s break down the differences, from scope to cost, and see how they fit into the UK construction scene.

Cat A, short for Category A, is the first stage of a fit-out, often called the “shell and core” upgrade. It’s about taking a newly built or refurbished building—basically a concrete box—and making it tenant-ready at a basic level. Think of it as the developer’s starting point. In a Cat A fit-out, you’re looking at essential infrastructure: raised floors, suspended ceilings, basic lighting, heating, ventilation, air conditioning (HVAC), and fire safety systems like sprinklers. The walls might get a basic coat of paint, and you’ll usually have power outlets installed, but that’s about as far as it goes. The space is functional but generic—there’s no branding, no partitions for offices, and definitely no fancy finishes. In the UK, Cat A is typically the landlord’s responsibility, ensuring the building meets basic standards, like those set by the British Council for Offices (BCO), so tenants can move in and customise it.

Cat B, or Category B, takes things to the next level. This is where the tenant steps in to make the space their own. If Cat A is the blank canvas, Cat B is the actual painting. It’s all about tailoring the space to the tenant’s specific needs, reflecting their brand and work culture. This stage includes installing partitions to create meeting rooms or open-plan areas, adding bespoke lighting, flooring (think carpets or hardwood), and detailed finishes like branded wall graphics or acoustic panels. Cat B also covers the tech side—think cabling for IT systems, Wi-Fi setup, and even furniture like desks and breakout area seating. In the UK, Cat B fit-outs are usually the tenant’s responsibility, though landlords might contribute through incentives like rent-free periods. The goal? A fully functional, move-in-ready office that screams “this is us.”

Cost is a big differentiator. Cat A fit-outs are generally cheaper because they’re more standardised. In London, a Cat A fit-out might cost between £40 and £60 per square foot, depending on the building’s location and quality. Cat B, on the other hand, can vary wildly based on the tenant’s vision. A high-end Cat B fit-out in a Central London office could run £80 to £150 per square foot, especially if you’re adding luxury finishes or advanced tech. For a 10,000-square-foot space, that’s a jump from £400,000–£600,000 for Cat A to £800,000–£1.5 million for Cat B. These costs reflect 2025 market rates, but they can fluctuate with material prices and labour shortages, which have been a challenge in the UK post-Brexit.

The timeline also differs. Cat A is quicker since it’s about the basics—think 8 to 12 weeks for a mid-sized project. Cat B takes longer, often 12 to 20 weeks, because it’s more detailed and involves coordination with designers, contractors, and sometimes even the tenant’s own team for things like IT setup. In the UK, where projects like the £106 billion HS2 are stretching resources, these timelines can stretch if skilled labour is hard to come by.

Another difference is the end user’s involvement. In Cat A, the tenant usually isn’t involved—the landlord or developer handles it to meet minimum standards. Cat B, though, is all about the tenant. They’ll work closely with designers and contractors to ensure the space fits their needs, from layout to aesthetics. This makes Cat B more collaborative but also more complex.

Sustainability is a growing focus in both stages, especially with the UK’s net-zero 2050 target. Cat A fit-outs now often include energy-efficient HVAC and LED lighting to meet regulations like Part L of the Building Regulations. Cat B lets tenants go further, adding eco-friendly materials or smart systems to cut energy use.

In short, Cat A gets a UK building to a basic, lettable standard, while Cat B transforms it into a bespoke workspace. They’re two sides of the same coin, balancing landlord responsibilities with tenant needs, costs, and timelines. Knowing the difference helps everyone—developers, tenants, and contractors—plan better and build smarter.