A practical, expert-led overview of Shell & Core, Cat A and Cat B fit outs — and what organisations need to know when planning a workplace in 2025.

Introduction

Understanding the terminology and stages of a commercial fit out is essential when planning a new workspace, yet terms like “Shell & Core”, “Cat A” and “Cat B” can feel unclear without the right context. This guide builds on that clarity immediately, offering a complete and practical explanation of each stage and how they influence budgets, timelines, design decisions and responsibilities.

Whether your organisation is assessing a potential new space, preparing an office relocation, or reviewing a landlord’s specification, this guide provides all the insights you need to make confident, informed decisions about your workplace project.

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What Is a Shell & Core Fit Out?

Shell & Core describes the base building structure delivered by the landlord or developer prior to any office space being made operational or occupier-ready.

It includes the building’s fundamental components — the “shell” (structure and envelope) and the “core” (shared areas, main risers and primary services).

The Shell Includes:

  • Structural frame (steel, concrete)

  • External façade and glazing

  • Roof and weathering systems

  • Building perimeter and cladding

  • External plant areas

The Core Includes:

  • Lift shafts and lifts

  • Staircases

  • WC blocks

  • Service risers

  • Fire protection zones

  • Plant rooms

  • Switch rooms

What Shell & Core Does Not Include:

  • Ceilings or raised floors

  • Meeting rooms

  • Internal walls or partitions

  • Occupation-ready lighting

  • Power distribution across floors

  • Decorative finishes

  • IT/data infrastructure

  • HVAC distribution into tenant areas

  • Kitchens, tea points or reception areas

In simple terms:
Shell & Core gives you the building. Cat A and Cat B create the workplace.

Shell & Core vs Cat A vs Cat B: Key Differences

Understanding the stages helps organisations accurately forecast budgets and responsibilities.

Cat A Fit Out

Typically delivered by the landlord after Shell & Core, Cat A creates a “blank but functional” interior. It is not yet tailored for occupation.

Includes:

  • Raised access floors

  • Suspended ceilings

  • Basic lighting

  • Basic mechanical & electrical distribution

  • Smoke detection (default layout)

  • Painted walls

  • Finished but unfurnished open-plan space

Cat A brings the space to a “ready to lease” standard.

Cat B Fit Out

Cat B is where the workplace becomes tailored to your organisation.

Includes:

  • Workplace design & strategy

  • Meeting rooms, collaboration spaces, focus booths

  • Reception areas

  • Breakout and tea points

  • Furniture and storage

  • Final mechanical & electrical modifications

  • Acoustic treatments

  • IT, AV and data infrastructure

  • Brand expression and finishes

This is the stage businesses invest in most heavily — because it shapes culture, productivity, comfort and brand identity.

Who Is Responsible for What at Shell & Core Stage?

Understanding responsibility helps avoid unexpected costs.

Landlords typically provide:

  • Structural integrity

  • External façade and weatherproofing

  • Main MEP plant and incoming services

  • Core fire safety infrastructure

  • Lifts, stairs and central WCs

  • Base-building documentation

Tenants are responsible for:

  • Internal layouts

  • Design, planning and Cat B works

  • IT, meeting rooms, acoustics and AV

  • Lighting, power and HVAC distribution

  • Workplace comfort and practical use

  • Ensuring compliance within their demise

These responsibilities must be clearly documented to maintain transparency between parties.

Why Understanding Shell & Core Matters

Businesses benefit from clear Shell & Core definitions because it impacts:

1. Budget Planning

The level of work required from Cat A and Cat B directly affects overall costs. Shell & Core space means more investment is needed to create a functional office.

2. Programme Timelines

Different landlords hand over space at different readiness levels. Understanding what is complete ensures fit out timelines are realistic.

3. Compliance and Safety

Fire strategy, accessibility and HSE obligations (referenced by the Health and Safety Executive and BCO best practice standards) can only be fully met when building parameters are clear.

4. Design Flexibility

Shell & Core spaces often allow greater freedom for bespoke workplace design, especially in hybrid or agile environments where layouts are more adaptable.

What’s Typically Left for Cat B After Shell & Core?

To help businesses visualise the transition from a base building to a finished workplace, here is what typically needs to be added during Cat B:

  • Power and data layouts

  • HVAC distribution (ducting, grilles, controls)

  • Feature lighting and task lighting

  • Meeting room structures

  • Breakout areas and kitchens

  • Flooring finishes

  • AV systems

  • Workstations and furniture

  • Acoustic treatments

  • Branding, graphics and touchpoints

This is why Shell & Core clarity is crucial — it determines the full scope of Cat B investment.

Shell & Core and Building Regulations

Shell & Core handovers must comply with UK building regulations and guidance such as:

  • HSE Workplace (Health, Safety and Welfare) Regulations 1992

  • The Building Regulations 2010

  • Fire Safety (England) Regulations 2022

  • BCO Guide to Specification (2019)

These standards shape building performance requirements, accessibility, fire safety, ventilation, and routes of escape.

Common Questions About Shell & Core

Is Shell & Core cheaper for landlords?

Yes — avoiding Cat A installations reduces upfront spend and gives tenants more flexibility.

Do tenants have any influence over Shell & Core design?

Only during development or lease negotiations. After handover, making changes can be expensive.

Does Shell & Core suit hybrid working?

Absolutely. It provides an adaptable foundation for custom Cat B layouts aligned with hybrid and flexible working strategies.

Can Cat A and Cat B overlap?

Yes, in some cases tenants negotiate to skip Cat A entirely and proceed directly to Cat B through a turnkey agreement — reducing waste and programme length.

How long does Shell & Core take to convert into a workplace?

Typical timelines vary based on size, but a 10,000 sq ft space may take:

  • Cat A: 4–8 weeks

  • Cat B: 8–20+ weeks

  • Existing building constraints may add or reduce time.

Free Resource: Shell & Core Fit Out Checklist

To support organisations planning their next project, here is a complete checklist covering:

Structure

  • Structural frame complete

  • Roof and weathering

  • Façade and glazing

  • Plant rooms and external areas

Core Areas

  • Lifts and stairs

  • WCs and circulation zones

  • Riser access

  • Fire safety elements

MEP Systems

  • Incoming power

  • Main HVAC plant

  • Sprinkler mains

  • Fire strategy

Documentation

  • O&M manuals

  • MEP drawings

  • Landlord responsibility matrix

Cat B Considerations

  • Workplace strategy

  • IT and network design

  • Power and HVAC distribution

  • Acoustics and meeting rooms

  • Furniture and layout

View your own shell and core PDF checklist here.

View Your PDF Checklist

Conclusion

Understanding Shell & Core is crucial for any organisation planning a fit out, refurbishment or relocation. It shapes budgets, technical decisions and the overall project direction — long before any design work begins.

A clear grasp of what the landlord provides versus what your team needs to deliver ensures projects run smoothly, safely and efficiently.

Shape Your Workplace with Expert Project Clarity

If you’d like help reviewing a potential space, understanding Shell & Core responsibilities or exploring Cat A vs Cat B planning, TB Fit Out offers friendly, expert guidance tailored to commercial organisations across the UK. Email sales@tbfitout.co.uk or call 01293 775509.