What does a commercial rewire really cost? This guide covers per-square-metre pricing for 3-phase distribution, containment systems, data cabling, fire alarm, and emergency lighting — with realistic figures for electricians pricing commercial contracts.
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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.
A commercial rewire typically costs £30 to £80 per square metre in 2026. Basic office work sits at £30 to £45/m²; a mid-spec rewire with 3-phase distribution, fire alarm and emergency lighting runs £45 to £60/m²; and a high-spec corporate fit-out reaches £60 to £80/m². Figures include containment, cabling, distribution, testing and certification to BS 7671:2018+A4:2026.
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A commercial rewire is a fundamentally different undertaking to a domestic rewire. The scale of distribution, the containment requirements, the regulatory framework, and the ancillary systems involved — fire alarm, emergency lighting, data infrastructure — mean that commercial electrical projects demand a different approach to pricing and project management.
Commercial rewires typically involve replacing the entire electrical installation from the incoming supply through to final circuits. This includes 3-phase distribution boards, sub-distribution, containment systems (cable tray, trunking, conduit), power circuits, lighting circuits, small power, and often fire alarm and emergency lighting systems.
Whether you are a building owner planning a refurbishment, a project manager tendering the electrical package, or an electrical contractor pricing a commercial rewire, this guide provides realistic per-square-metre costs based on current UK market rates.
Written by a qualified electrician and reviewed against BS 7671:2018+A4:2026.
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Commercial electrical installations are typically priced per square metre of gross internal floor area. The rate varies significantly depending on the specification level and building complexity.
Indicative UK market guidance — not a quote. Rates cover supply, install, test and certification.
Standard office rewire with power, lighting and basic containment. No fire alarm or emergency lighting in scope. Single distribution board replacement. Suits small refurbishments under 300m².
Full rewire with 3-phase distribution, sub-distribution boards, structured data cabling, fire alarm (Category L2 to BS 5839-1), emergency lighting (BS 5266-1) and dado or floor-box containment.
MCCB panel board, multiple sub-distribution boards, high-density power and data, addressable fire alarm, maintained emergency lighting, DALI lighting control, BMS integration and UPS provisions. Corporate offices, medical centres, high-end retail.
| Floor area | Specification | Indicative cost |
|---|---|---|
| 200m² | Basic office | £6,000–£9,000 |
| 500m² | Mid spec | £22,500–£30,000 |
| 1,000m² | High spec fit-out | £60,000–£80,000 |
These figures include materials, labour, containment, testing and certification. They exclude specialist systems such as access control or CCTV unless specifically scoped, and assume a vacant or partly occupied building. Always treat per-m² rates as a starting point for a measured estimate, not a fixed price — the factors covered further down this guide can move the rate by a factor of two.
Almost all commercial premises operate on a 3-phase supply. The distribution architecture is the backbone of the installation and must be designed to accommodate the assessed load with appropriate diversity applied.
| Item | Supply & install |
|---|---|
| 3-phase main distribution board (TPN)An 18-way TPN board with MCCB incomer is roughly £3,500 installed; rate scales with rating and number of ways. | £2,000–£6,000 |
| Sub-distribution board (each)Each floor or zone typically has its own board fed from the main board via a submain cable. | £800–£2,500 |
| Submain cables (per metre)Depends on CSA. A 4-core 25mm² SWA floor submain is roughly £25/m installed including containment and termination. | £15–£60/m |
| Surge protection (SPD)Type 1+2 at the main board £300–£600 installed; Type 2 at sub-boards £150–£300 each. See the regulations section for when these are required. | £150–£600 |
Phase balancing is critical in commercial installations. The design should distribute single-phase loads evenly across the three phases to avoid excessive neutral current and voltage imbalance. This must be verified during commissioning.
Commercial fit-outs increasingly include EV charging provision. BS 7671:2018+A4:2026 Section 722 covers electric vehicle charging installations, and Regulation 722.311.201 permits load curtailment — load reduction or disconnection, automatic or manual — to be taken into account when determining the maximum demand of the installation. This means that multiple EV charging points can often be accommodated on an existing incoming supply provided a suitable load management system is specified, without requiring a costly DNO supply upgrade. Designers must document the curtailment assumptions used in the demand calculation. Budget an additional £800 to £2,500 per charge point installed, plus allowance for the load management controller where multiple points are provided.
Containment is one of the biggest cost drivers in commercial electrical work. Unlike domestic installations where cables are clipped or run in the building fabric, commercial work requires engineered containment routes that provide access, support, and fire protection.
Cable tray (£8 to £15/m installed) and cable basket (£6 to £12/m installed) are the primary containment for distribution routes above suspended ceilings. Medium-duty cable tray is used for power cables; cable basket is often used for data cables. Fire barriers must be installed where tray passes through compartment walls.
Dado trunking (£12 to £25/m installed) provides power and data outlets at desk height in offices. Floor trunking and floor boxes (£80 to £200 per floor box installed) serve open-plan areas. Steel conduit (£8 to £18/m installed) is used in exposed areas and where mechanical protection is needed.
Containment typically accounts for 25% to 40% of the total electrical installation cost in commercial buildings. Getting the containment design right at tender stage is critical — underestimating containment is one of the most common causes of losses on commercial electrical contracts.
A commercial rewire scope frequently includes structured data cabling, fire alarm systems, and emergency lighting. These are often the responsibility of the electrical contractor even though they require specialist design competence.
| System | Indicative cost |
|---|---|
| Structured data cabling (Cat6A point)Includes patch-panel termination and testing. A typical desk needs 2 points. Fibre backbone between comms rooms: £500–£1,500 per link. | £80–£150/point |
| Fire alarm to BS 5839-1 (Category L2, addressable)Detectors, manual call points, sounders, interface units and an addressable panel. Design must be by a competent fire alarm designer. | £6–£12/m² |
| Emergency lighting to BS 5266-1Maintained or non-maintained luminaires on escape routes, open areas and high-risk task areas. 3-hour duration is standard for most commercial premises. | £3–£6/m² |
For a 500m² office, ancillary systems might add £8,000 to £15,000 to the base electrical installation cost. These systems require separate design, commissioning certificates, and ongoing maintenance contracts.
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The per-square-metre rate can vary by a factor of two or more depending on these key variables:
A vacant shell with clear ceiling voids is far cheaper to wire than an occupied building with limited access, asbestos risks and out-of-hours working.
Outlet density varies hugely. A basic warehouse office might have 1 double socket per 10m²; a trading floor might have 4 doubles per desk at 6m² per desk.
Occupied buildings may need evening and weekend working, adding 25% to 50% to labour costs through overtime rates and reduced productivity.
Pre-2000 buildings often contain asbestos. Removal or encapsulation is the client's cost but causes programme delays that affect the electrical contractor.
London rates are 20% to 40% higher than regional rates due to labour costs, parking, congestion charge and site-access restrictions.
Commercial electrical installations must comply with BS 7671:2018+A4:2026 and the Electricity at Work Regulations 1989. Unlike domestic work, Part P of the Building Regulations does not apply to commercial premises — but the installation must still comply with the relevant parts of the Building Regulations, particularly Part B (fire safety).
An Electrical Installation Certificate (EIC) must be issued on completion, signed by the designer, installer, and inspector/tester. For larger installations, the testing and inspection may be split across multiple schedules of test results.
Surge protection is a key consideration on commercial rewires. BS 7671:2018+A4:2026 redrafted Regulation 443.4, and Regulation 443.4.1 now requires protection against transient overvoltages where the consequence of an overvoltage could result in:
For all other cases, protection against transient overvoltages shall be provided unless the owner of the installation declares it is not required due to any loss or damage being tolerable and they accept the risk of damage to equipment and any consequential loss. Limb (b) of Regulation 443.4.1 was deleted by the BS 7671:2018+A2:2022 Corrigendum (May 2023).
On most commercial premises the "significant financial or data loss" condition is readily met, so in practice a Type 1+2 SPD at the main distribution board is specified on the large majority of commercial rewires. Where SPDs are used they must be selected and erected to Section 534. If the protective distance between an SPD and the equipment it protects exceeds 10 metres, Regulation 534.4.4.2 warns that oscillations can lift the voltage at the equipment terminals to up to twice the SPD's voltage protection level — so additional coordinated Type 2 SPDs are typically fitted closer to downstream sub-distribution boards and sensitive equipment.
Additional protection by a 30mA RCD is required for socket-outlets with a rated current not exceeding 32A under Regulation 411.3.3 of BS 7671. In commercial installations this is typically delivered by RCBOs on individual circuits rather than bank RCDs, so that a fault on one circuit does not take out several others — the same approach the regulations encourage to avoid unwanted tripping.
Fire alarm systems must be designed and installed to BS 5839-1, with a separate commissioning certificate. Emergency lighting must comply with BS 5266-1. Both systems require ongoing periodic testing and maintenance.
Commercial rewires are high-value contracts with significant profit potential — but also significant risk if priced incorrectly. Here are practical tips for quoting commercial electrical work:
Walk the building and measure every containment route before pricing anything else. Containment is the biggest variable cost. Use Elec-Mate's quoting app to build itemised schedules of containment, distribution equipment, and final circuits.
Allow adequate time for testing and completing the EIC. A 500m² commercial installation might take 2 to 3 days to test and certify properly. Do not squeeze this into the last afternoon of the contract.
Include programme-related costs (prelims): site supervision, welfare, temporary lighting, tool hire, waste disposal, and site access. On a 6-week commercial contract, prelims can add 8% to 12% to the direct costs.
Elec-Mate's quoting app handles itemised commercial tenders with distribution equipment, containment schedules, and final circuit costing.
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