Office fit-out electrics cover the power, lighting, data containment, fire alarm and emergency lighting that turn a bare shell or tired floor into a working office. The electrical scope depends on whether you are dealing with a Cat A base build or a Cat B tenant fit-out, and because the electrics run alongside partitioning, ceilings and mechanical services, coordination matters from day one. This guide explains what is involved and how to plan it properly.
What is the difference between Cat A and Cat B?
A Cat A fit-out is the landlord’s base build. It typically includes the incoming supply and distribution, a suspended ceiling with general lighting, raised floors, basic mechanical services and finished walls, ready for a tenant to make their own.
A Cat B fit-out is the tenant’s layout. Electrically, it adds the small power, floor boxes, data outlets, feature lighting, power feeds to partitions and furniture, comms room supplies and kitchen or tea point circuits that suit a particular business.
The split matters because the Cat A distribution may need extending or upgrading before Cat B circuits can be added. Planning containment routes and spare capacity for both stages from the start avoids expensive rework later.
What electrical work is included?
- Small power: floor boxes, dado or desk trunking, and dedicated circuits for printers, kitchen equipment and comms racks.
- Lighting: LED panels or linear systems, lighting controls such as presence and daylight sensing, and emergency lighting to BS 5266.
- Data and containment: cable tray, basket and trunking routes coordinated with partitions and the ceiling grid.
- Fire detection and alarm: alterations to BS 5839 where the layout changes or new rooms are created.
- Distribution: new or extended distribution boards sized for the final load.
- Testing and certification: an Electrical Installation Certificate for new circuits, or a Minor Electrical Installation Works Certificate for small alterations, plus updated fire alarm and emergency lighting certificates.
How do you plan power, lighting and data?
Start with the furniture layout and headcount. Desk positions drive floor box and dado positions, not the other way round, so the electrical design should follow a fixed desk plan.
Agree the containment strategy early so electrical, data and mechanical routes do not clash above the ceiling or below the floor. This is where having one contractor coordinating several trades saves time and rework.
Size distribution for the final Cat B load, not just day-one occupancy, and leave spare ways for future changes. Confirm the capacity of the incoming supply before ordering equipment, especially if you are adding comms rooms, kitchens or EV charging.
Which regulations and standards apply?
- BS 7671, the IET Wiring Regulations, governs the design, installation and testing of the electrical installation. New work should now be to BS 7671:2018+A4:2026.
- BS 5839-1 covers fire detection and alarm systems, and BS 5266 covers emergency lighting.
- Part L of the Building Regulations sets energy efficiency requirements for new or replacement fixed lighting and controls.
- CDM 2015 sets out client, designer and contractor duties on all construction projects, with larger projects also needing to be notified to HSE.
Before work starts, check whether building regulations approval is needed, for example where alterations affect fire safety or escape routes, and get landlord consent where the lease requires it.
How long does a fit-out take, and what causes delays?
A small single-floor refit can take a few weeks, while larger or multi-floor schemes can take several months.
Long-lead items are the most common cause of slippage. Switchgear, specialist lighting and comms equipment can have extended delivery times, so order them early.
Late design changes after first fix are expensive, because containment and cabling are already in. A single point of accountability across electrical, mechanical and building fabric work reduces the gaps between trades and keeps the programme on track.
What should you ask a fit-out electrical contractor?
- Who employs the electricians, and who supervises them on site?
- Which certificates will be issued at completion, and in what format?
- How will you handle landlord approvals and any building regulations requirements?
- How will you coordinate with the partitioning, ceiling and mechanical trades, and who owns the programme?
- What support is available after handover, such as planned maintenance and reactive cover?
Frequently asked questions
How much does an office fit-out cost?
There is no reliable rule of thumb for the electrical element, because scope, specification and the condition of the existing installation drive the figure. A site survey is the sensible starting point.
Do I need an EICR before a fit-out?
It is often worth it. An inspection of the existing installation shows whether the distribution and wiring can be reused, and avoids surprises once work has started.
Can the office stay open during the work?
Often, yes. Phased working and out-of-hours isolation can keep parts of the building running, but this needs planning into the programme from the start.
Key takeaways
- Cat A is the landlord’s base build, while Cat B is the tenant’s layout, small power, data and feature lighting.
- Fit-out electrics cover small power, lighting and controls, emergency lighting, containment, fire alarm alterations and certification.
- BS 7671, BS 5839-1, BS 5266, Part L and CDM 2015 all apply.
- Plan from a fixed desk layout and size distribution for the final load.
- Order long-lead equipment early and avoid design changes after first fix.
Delta First handles the electrical side of office fit-outs through its commercial electrical contracting service, alongside mechanical and building fabric work. After handover, commercial electrical maintenance and periodic EICR testing keep the finished office safe and compliant.
Talk to Delta First
Delta First Building Services is a Colchester-based M&E contractor working across Essex, Suffolk and London, with commercial work in Norfolk. Our directly employed electrical team is supported by vetted specialist subcontractors where extra certification is needed, and we offer genuine 24/7 emergency cover. Get in touch to arrange a survey or quote.