What regulations apply to an office electrical fit-out in the UK?

An office electrical fit-out must comply with multiple overlapping regulatory requirements. BS 7671:2018+A4:2026 (the IET Wiring Regulations) provides the primary electrical installation standard. Building Regulations Part L2B (energy efficiency for existing buildings other than dwellings) applies to the lighting and electrical services installation. Part B (fire safety) applies to all fire-related systems. The Electricity at Work Regulations 1989 impose duties on the occupier to maintain electrical systems safely. Where the tenant is taking a Category B fit-out in a leased building, the landlord's fit-out guide will specify additional requirements for the electrical installation that must be met alongside the statutory standards.

How many power outlets are needed in an office electrical fit-out?

There is no statutory minimum number of outlets for office use, but CIBSE guidance and good practice suggest a minimum of 2 twin socket outlets per workstation for a standard office layout. Open-plan offices typically have a floor socket arrangement with a perimeter and column trunking strategy providing flexible socket coverage. High-density offices with hot-desking may require higher outlet densities. In addition to workstation power, separate circuits should be provided for: kitchen and breakout areas, AV and presentation equipment, server room or comms room, and building management systems. All socket outlets must be protected by 30mA RCDs per Regulation 411.3.3 of BS 7671.

What illuminance level is required for office lighting in the UK?

CIBSE Lighting Guide LG7 (Offices) recommends a maintained average illuminance (Em) of 300 to 500 lux at the working plane (0.7m above floor level) for general office tasks. Screen-based tasks require careful glare control — the Unified Glare Rating (UGR) must not exceed 19 for computer-dominated offices to prevent reflections on screens. Part L of the Building Regulations requires that office lighting achieves a minimum lighting efficacy (the LENI — Lighting Energy Numeric Indicator) target, which is met by using LED luminaires with automatic controls (daylight dimming and presence detection). Compliance with LG7 and Part L must be demonstrated by a photometric design study (DIALux or equivalent).

What fire alarm category is required for an office building?

The fire alarm category for an office building is determined by the fire risk assessment carried out in accordance with the Regulatory Reform (Fire Safety) Order 2005. For most office buildings, a Category L2 or L3 automatic fire detection system is specified (automatic detection in defined areas of highest risk, with manual call points and sounders throughout). The fire alarm system must comply with BS 5839-1 for design, installation, commissioning, and maintenance. Category M systems (manual alarm only, no automatic detection) are only appropriate for small offices where all occupants can observe a fire as it starts. The fire risk assessment will determine the category required for each specific building.

Is EV charging required for new office developments in the UK?

Building Regulations Part S (Infrastructure for the Charging of Electric Vehicles, applicable in England) requires that new office buildings with more than 10 car parking spaces must provide at least one active EV charge point and cable routes for at least one in five of all remaining spaces. For major refurbishments of existing buildings with more than 10 spaces, at least one active charge point and cable routes for one in five remaining spaces must be provided. The EV charge points must comply with BS 7671 Section 722 and the Electric Vehicles (Smart Charge Points) Regulations 2021 (smart functionality required for all new private charge points). Scotland and Wales have their own equivalent regulations.

What sub-metering is required in an office fit-out under Part L?

Building Regulations Part L2B (existing non-domestic buildings) requires that where more than 1,000 m² of the building is being renovated, sub-metering must be provided so that energy use can be monitored and managed. For office fit-outs this typically requires separate energy meters for: lighting, small power (sockets and equipment), heating and cooling, and any significant process loads. Where an office building has multiple tenants, sub-metering is required to allow each tenant's energy consumption to be separately measured. Automatic Meter Reading (AMR) systems that automatically collect and transmit meter data are preferred for buildings above a certain size to facilitate compliance with the Energy Savings Opportunity Scheme (ESOS).

What is a Category A and Category B office fit-out?

In UK commercial property, office fit-out is typically described in two categories. A Category A (Cat A) fit-out is the base building finish provided by the landlord before a tenant takes occupation. It typically includes: raised access floors, suspended ceilings, basic mechanical and electrical services (lighting, ventilation, perimeter power trunking), and decorated walls. The electrical installation at Cat A stage will have a main distribution board, perimeter power trunking, basic general lighting, emergency lighting, and fire alarm with detector heads in the ceiling void. A Category B (Cat B) fit-out is the tenant's own fit-out of their space, adding workstation power, data and AV installations, feature lighting, kitchen and breakout areas, and any specialist systems such as a secure server room or AV conference suite.

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Office Electrical Fit Out UK: Commercial Office Wiring Guide

The complete technical guide to office electrical fit-out in the UK — covering power distribution, data and AV points, office lighting design to CIBSE LG7, emergency lighting to BS 5266-1, fire alarm integration, EV charging, and energy metering for Building Regulations Part L compliance.

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14 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

1Office electrical fit-outs must comply with BS 7671:2018+A4:2026, Building Regulations Part L (energy efficiency) and Part B (fire safety), BS 5839-1 (fire detection and alarm), and BS 5266-1 (emergency lighting). Planning permission may be required for Category B fit-outs in leased premises.
2UK office lighting design should target 300 to 500 lux maintained average illuminance at the working plane (0.7m above floor) in accordance with CIBSE Lighting Guide LG7 (Offices). Automatic daylight dimming and occupancy control are required by Part L of the Building Regulations.
3Emergency lighting in offices must comply with BS 5266-1 and provide a minimum of 1 lux on escape routes and 0.5 lux in open areas for a minimum duration of 1 hour (or 3 hours for certain high-risk areas).
4All new office small power socket outlets must be protected by 30mA RCDs in accordance with Regulation 411.3.3 of BS 7671. Type A RCDs are suitable for most office circuits; Type B RCDs are required where equipment with DC residual current components may be connected.
5Part L of the Building Regulations requires sub-metering of significant energy uses in office buildings. Energy Performance Certificates (EPCs) are required for offices offered for sale or letting, and Display Energy Certificates (DECs) are required for public buildings over 250 m².

01 · Commercial Electrical Guide

Office Electrical Fit-Out in the UK

Office electrical fit-outs represent a significant and consistent market for UK electrical contractors. Every office lease event — a new building completion, a tenant change, or a refurbishment — generates substantial electrical work. The rise of hybrid working has also driven a wave of office redesigns, from traditional cellular layouts to open-plan collaborative environments, each with different power, data, and lighting requirements.

Office electrical fit-outs involve a wide range of systems: power distribution from the incoming supply to individual workstations, structured data cabling and AV systems, high-quality LED lighting with daylight and occupancy controls, emergency lighting, fire alarm integration, EV charging in the car park, and energy metering for Part L compliance. Co-ordination between the electrical contractor, mechanical contractor, data/AV specialist, and fire alarm specialist is essential.

Key regulations: BS 7671:2018+A4:2026, Electricity at Work Regulations 1989, Building Regulations Parts B, L, and S, BS 5839-1 (fire detection), BS 5266-1 (emergency lighting), CIBSE LG7 (office lighting), Electric Vehicles (Smart Charge Points) Regulations 2021.
Certification: All office electrical fit-out work must be notified to Building Control and certified with an Electrical Installation Certificate on completion. Where the electrical contractor is registered with a competent person scheme (NICEIC, NAPIT, ELECSA), self-certification is available without separate Building Control notification.

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02 · Commercial Electrical Guide

Power Distribution in Office Fit-Outs

Power distribution in an office fit-out must supply workstation power, kitchen and breakout areas, AV and presentation equipment, server and comms rooms, and building services — all from a distribution system that is flexible enough to accommodate future layout changes without major rewiring.

Distribution boards: A sub-main distribution board serves each floor or zone of the office. Sub-main cables from the main distribution board feed these zone boards, which house the MCBs protecting individual final circuits. Zone boards should include spare ways (minimum 25 per cent) for future additions and should be located in accessible positions, typically within server/comms rooms or dedicated electrical risers.
Perimeter trunking: In Cat A office fit-outs, perimeter trunking (three-compartment: power, data, and open) is typically installed around the office perimeter at desk height. Each trunking section is supplied by a radial final circuit, and socket outlets and data outlets can be added or relocated by the tenant during their Cat B fit-out.
Floor boxes: Open-plan office layouts use floor boxes in the raised access floor to provide power and data at workstations in the centre of the floor plate (away from perimeter trunking). Floor boxes must be rated for the load they carry and must be sealed against the ingress of water during floor cleaning.
RCD protection: All socket outlet circuits in the office must be protected by 30mA RCDs in accordance with Regulation 411.3.3 of BS 7671. RCBOs (combined MCB and RCD) are commonly used in office distribution boards to provide individual circuit RCD protection without shared tripping of multiple circuits.

03 · Commercial Electrical Guide

Data and AV Points in Office Fit-Outs

Although structured data cabling is typically a separate specialist trade, the electrical contractor must co-ordinate closely with the data installer to ensure containment, power supplies, and earthing arrangements are correctly provided. AV systems in conference rooms and presentation areas also require specialist electrical provisions.

Structured cabling: Cat6A or Cat6 structured cabling is the standard for new office data installations in the UK, supporting 10 Gigabit Ethernet to the desktop. Optical fibre (OS2 single-mode or OM4 multimode) is used for backbone connections between communication rooms. The electrical contractor provides the containment (cable tray, trunking, conduit) and the data contractor pulls and terminates the cabling.
Power over Ethernet (PoE): IP phones, wireless access points, CCTV cameras, and access control readers increasingly use PoE (Power over Ethernet) for both power and data via a single Cat6A cable. The network switch provides the PoE power. The electrical contractor must ensure the switch cabinet (patch cabinet) has adequate power supply (typically a dedicated 16A or 32A circuit) to support the PoE budget of all connected devices.
Conference room AV: Conference rooms require a dedicated power zone for AV equipment including projectors, large-format displays, videoconferencing systems, and audio amplifiers. AV equipment should be on a separate circuit from general power to prevent interference from other loads. Floor boxes or wall outlets for AV should be positioned to avoid cable trip hazards.
Server/comms room: The IT comms room requires a dedicated power supply (often a UPS-protected circuit), adequate electrical capacity for current and future rack power densities, precision air conditioning, and appropriate fire suppression. The electrical installation in the comms room should be designed to the same principles as a small data centre.

04 · Commercial Electrical Guide

Office Lighting Design

High-quality lighting is one of the most visible and occupant-affecting aspects of an office fit-out. Good lighting design improves productivity, reduces eye strain, and creates the right environment for different types of work. UK offices must comply with CIBSE LG7 and Part L of the Building Regulations.

Illuminance levels: CIBSE LG7 specifies 300 to 500 lux maintained average at the working plane for general office work. Task areas, drawing offices, and precision work may require up to 750 lux. Circulation areas and corridors require 100 to 150 lux. Reception areas typically require 200 to 300 lux.
Glare control: Computer screen glare is a major comfort issue in offices. LED luminaires must be specified with a UGR (Unified Glare Rating) not exceeding 19 for screen-based work environments. Indirect or semi-indirect luminaires (uplighting or louvre fittings) are preferred over direct recessed downlighters in open-plan offices.
Lighting controls: Part L of the Building Regulations requires automatic lighting controls in offices, including: presence detection (switching off lights in unoccupied areas automatically), daylight dimming (dimming or switching lights near windows in response to available daylight), and local manual override. DALI control systems are widely used for office lighting, allowing individual luminaire addressing and integration with the BMS.
Tunable white lighting: Human-centric lighting (HCL) systems that vary the colour temperature of the office lighting throughout the day (cooler, bluer light in the morning to promote alertness; warmer light in the afternoon to reduce fatigue) are increasingly specified in high-quality office fit-outs. These systems require DALI or DALI-2 control with tunable white (Tw) luminaires.

05 · Commercial Electrical Guide

Emergency Lighting in Office Buildings

Emergency lighting in offices must ensure that all occupants can safely evacuate the building in the event of a mains power failure, and that security and safety systems can continue to operate. BS 5266-1 and Regulation 560.7 of BS 7671 govern the design, installation, and testing of emergency lighting.

Escape route lighting: Escape route luminaires must provide a minimum maintained illuminance of 1 lux at floor level along the centreline of the escape route, with a uniformity ratio (minimum to average) of not less than 1:40. Luminaires must be positioned at every change of direction, at every exit door, at stairways (at each landing and at each change of direction), and at every fire call point and first-aid station.
Anti-panic lighting: In open-plan office areas (open areas greater than 60 m²), anti-panic lighting must provide a minimum of 0.5 lux at floor level throughout the area (avoiding fixed obstacles) so that occupants can safely move towards escape routes.
Duration: Emergency lighting in offices must maintain the required illuminance for a minimum duration of one hour. Where the premises are used for entertainment or public gatherings, a minimum of three hours is required. Self-contained maintained luminaires with integrated batteries, or a central battery system, are both acceptable approaches.
Automated testing: BS 5266-1 requires monthly functional testing and annual full-duration testing of emergency lighting. Automated self-testing emergency luminaires (addressable self-test systems) simplify compliance by automatically performing tests and generating electronic records, reducing the labour cost of manual testing in large office buildings.

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06 · Commercial Electrical Guide

Fire Alarm Integration in Office Fit-Outs

The fire alarm installation is closely integrated with the office electrical fit-out. Electrical contractors must understand the interface between the fire alarm system and the building's electrical services, even where the fire alarm installation itself is carried out by a specialist contractor.

Fire-resistant cable: Fire alarm cabling must use fire-resistant cable complying with BS 7629 or equivalent, maintaining circuit integrity in fire conditions. Regulation 521.10 of BS 7671 requires fire-resistant wiring systems for safety services. In an office fit-out, the fire alarm contractor typically supplies and installs the fire alarm cable; the electrical contractor provides the power supply circuit to the fire alarm control panel.
Electrical interfaces: The fire alarm system typically interfaces with the building's electrical services to trigger automatic actions on detection, including: releasing fire door magnetic holders, activating smoke extraction fans, shutting down air handling units (to prevent smoke spread), and sending signals to lifts to call them to the ground floor.
Power supply to the fire alarm panel: The fire alarm panel must be supplied from a dedicated circuit taken directly from the distribution board, protected by a dedicated MCB clearly labelled "FIRE ALARM — DO NOT SWITCH OFF". This circuit must not be connected to a general power distribution circuit that could be switched off by a building occupant.

07 · Commercial Electrical Guide

EV Charging in Office Car Parks

Building Regulations Part S requires EV charging provision for new office buildings and major refurbishments with car parking. Even where Part S does not apply, office occupiers increasingly demand EV charging as a workplace benefit, and it is becoming a standard element of office fit-outs.

Part S requirements: For new office buildings with more than 10 car parking spaces, Building Regulations Part S (England) requires at least one active EV charge point and cable routes suitable for charge points for at least one in five of all remaining spaces. The active charge point must be smart (complying with the Electric Vehicles (Smart Charge Points) Regulations 2021).
BS 7671 Section 722: EV charging installations must comply with Section 722 of BS 7671. Key requirements include: 30mA Type B RCD protection for each charge point (or Type A where the manufacturer confirms no DC residual current exceeds 6mA); an isolating switch for each charge point; and appropriate earthing arrangements, including protective earth monitoring where required by the charge point manufacturer.
Load management: Where multiple charge points are installed, a dynamic load management system should be provided to prevent the aggregate EV charging demand from overloading the office building's incoming supply. The load management system monitors available capacity headroom and distributes charging power among active charge points.

08 · Commercial Electrical Guide

Energy Metering and Part L Compliance

Energy efficiency and sub-metering requirements for office electrical fit-outs are governed by Part L of the Building Regulations. Compliance must be demonstrated at the design stage and verified on completion.

Part L2B sub-metering: Where more than 1,000 m² of a building is being renovated, Part L2B requires sub-meters for lighting, small power, HVAC, and any significant process loads. In a Cat B office fit-out, the tenant should install sub-metering that allows their energy use to be separately measured from the base building energy (landlord's common areas and services).
Lighting energy: Part L requires that office lighting achieves a target LENI (Lighting Energy Numeric Indicator) value. The LENI is calculated by the lighting designer using a standardised methodology and must meet the target value specified in Part L. LED luminaires with automatic controls (presence detection and daylight dimming) are essential for meeting LENI targets.
Energy Performance Certificates: An EPC is required for any office building offered for sale or let (since October 2008). The EPC rating (A++ to G) is based on the asset rating of the building's fixed services, including the electrical installation and lighting. From April 2023, office buildings with an EPC rating below E cannot be let (MEES — Minimum Energy Efficiency Standards). From 2027, the minimum will rise to D, and from 2030 to C.

09 · Commercial Electrical Guide

For Electricians: Office Electrical Fit-Out Contracting

Office electrical fit-out is a consistently active market for UK electrical contractors. The regular churn of office leases, the expansion of new office developments in UK cities, and the wave of post-pandemic office redesigns all generate significant contracting opportunities.

Working in Occupied Buildings

Office refurbishments are frequently carried out in occupied buildings, requiring work to be phased to minimise disruption to tenants. Safe isolation procedures are particularly important in occupied buildings — always confirm the exact circuit to be isolated with the building manager before switching off, and use lockout devices to prevent inadvertent re-energisation.

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Frequently Asked Questions — Office Electrical Fit Out

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Office Electrical Fit Out UK: Commercial Office Wiring Guide

Key Takeaways

Office Electrical Fit-Out in the UK

Get the BS 7671 A4:2026 Cheat Sheet — free

Power Distribution in Office Fit-Outs

Data and AV Points in Office Fit-Outs

Office Lighting Design

Emergency Lighting in Office Buildings

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Fire Alarm Integration in Office Fit-Outs

EV Charging in Office Car Parks

Energy Metering and Part L Compliance

For Electricians: Office Electrical Fit-Out Contracting

Working in Occupied Buildings

Complete Certification on Site

Manage office fit-out electrical projects with Elec-Mate

Frequently Asked Questions — Office Electrical Fit Out

What regulations apply to an office electrical fit-out in the UK?

How many power outlets are needed in an office electrical fit-out?

What illuminance level is required for office lighting in the UK?

What fire alarm category is required for an office building?

Is EV charging required for new office developments in the UK?

What sub-metering is required in an office fit-out under Part L?

What is a Category A and Category B office fit-out?

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