What does a commercial rewire really cost? Per-square-metre pricing for 3-phase distribution, containment, data cabling, fire alarm and emergency lighting — with the BS 7671:2018+A4:2026 regulations that drive the price.
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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.
The short answer
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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01 · Cost Guide
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 gives realistic per-square-metre costs alongside the BS 7671 requirements that drive them.
Written by a qualified electrician. Regulation references verified against BS 7671:2018+A4:2026.
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02 · Cost Guide
Commercial electrical installations are priced per square metre of gross internal floor area. Three specification bands cover most jobs.
Indicative UK market guidance, not a quote. Rates cover supply, install, test and certification.
| Specification | Rate | What it buys |
|---|---|---|
| Basic | £30–£45/m² | Power, lighting and basic containment. No fire alarm or emergency lighting in scope. Single distribution board replacement. Suits refurbishments under 300m². |
| MidMost common | £45–£60/m² | 3-phase distribution, sub-distribution boards, structured data cabling, fire alarm (Category L2 to BS 5839-1), emergency lighting to BS 5266-1, and dado or floor-box containment. |
| High spec | £60–£80/m² | 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. Treat a per-m² rate as the starting point for a measured estimate, never a fixed price — the factors further down this guide can move the rate by a factor of two.
03 · Cost Guide
Almost all commercial premises operate on a 3-phase supply. The distribution architecture is the backbone of the installation and must be designed to carry the assessed maximum demand with appropriate diversity applied.
| Item | Cost | Notes |
|---|---|---|
| 3-phase main board (TPN) | £2,000–£6,000 | An 18-way TPN board with MCCB incomer is roughly £3,500 installed. The rate scales with rating and number of ways. |
| Sub-distribution board (each) | £800–£2,500 | Each floor or zone typically has its own board fed from the main board via a submain cable. |
| Submain cables (per metre) | £15–£60/m | Depends on CSA. A 4-core 25mm² SWA floor submain is roughly £25/m installed including containment and termination. |
| Surge protection (SPD) | £150–£600 | 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. |
The design should distribute single-phase loads evenly across the three phases to limit neutral current and voltage imbalance. This is verified during commissioning and recorded on the schedules that accompany the certificate.
Commercial fit-outs increasingly include EV charging. Section 722 of BS 7671:2018+A4:2026 covers electric vehicle charging installations, and Regulation 722.311.201 states that load curtailment — load reduction or disconnection, either automatically or manually — may be taken into account when determining the maximum demand of the installation or part thereof. In practice that means several charge points can often be accommodated on the existing incoming supply where a suitable load management system is specified, avoiding a costly DNO supply upgrade. The regulation does not prescribe a calculation method, so the designer must document the curtailment assumptions behind the demand figure. Budget £800 to £2,500 per charge point installed, plus the load management controller where multiple points are provided.
04 · Cost Guide
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.
| Containment | Rate | Typical use |
|---|---|---|
| Cable tray | £8–£15/m | Primary power distribution routes above suspended ceilings. Medium-duty tray is the commercial default. |
| Cable basket | £6–£12/m | Data cabling routes and lighter secondary distribution. |
| Dado trunking | £12–£25/m | Power and data outlets at desk height in cellular offices. |
| Floor boxes | £80–£200 each | Open-plan areas fed from floor trunking or an access floor void. |
| Steel conduit | £8–£18/m | Exposed and industrial areas, and anywhere mechanical protection is needed. |
Fire barriers must be reinstated wherever containment passes through a compartment wall or floor. Containment typically accounts for 25% to 40% of the total electrical installation cost in commercial buildings, and underestimating it is one of the most common causes of losses on commercial electrical contracts. Measure every route before you price anything else.
05 · Cost Guide
A commercial rewire scope frequently includes structured data cabling, fire alarm and emergency lighting. These usually sit with the electrical contractor even though each requires its own design competence and its own commissioning certificate.
| System | Indicative cost | Scope |
|---|---|---|
| Structured data cabling (Cat 6A) | £80–£150/point | Includes patch-panel termination and testing. A typical desk needs 2 points. Fibre backbone between comms rooms adds £500–£1,500 per link. |
| Fire alarm, BS 5839-1 (Category L2, addressable) | £6–£12/m² | Detectors, manual call points, sounders, interface units and an addressable panel. Category follows from the fire risk assessment. |
| Emergency lighting, BS 5266-1 | £3–£6/m² | Maintained or non-maintained luminaires on escape routes, open areas and high-risk task areas. 3-hour duration is the usual commercial specification. |
For a 500m² office, ancillary systems typically add £8,000 to £15,000 to the base electrical installation cost.
BS 7671:2018+A4:2026 introduced a new Section 716 covering the distribution of ELV DC power over balanced data cabling — PoE lighting, wireless access points, cameras and door controllers. If your scope includes any of those, the data cabling package is now partly a BS 7671 package. The headline requirements are:
The practical cost impact is on connectors and on bundle planning: cheap patch panels that will not carry 750 mA per contact are no longer an acceptable saving on a PoE-fed installation.
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06 · Cost Guide
The per-square-metre rate can vary by a factor of two or more depending on these variables.
| Factor | Effect on the rate |
|---|---|
| Building type and access | A vacant shell with clear ceiling voids is far cheaper to wire than an occupied building with limited access, asbestos risk and out-of-hours working. |
| Specification density | Outlet density varies hugely. A basic warehouse office might have one double socket per 10m²; a trading floor might have four doubles per desk at 6m² per desk. |
| Working-hours restrictions | Occupied buildings may need evening and weekend working, adding 25% to 50% to labour costs through overtime rates and reduced productivity. |
| Asbestos and legacy services | Pre-2000 buildings often contain asbestos. Removal or encapsulation is the client's cost, but the programme delay lands on the electrical contractor. |
| Location | London rates run 20% to 40% above regional rates on labour, parking, congestion charge and site-access restrictions. |
07 · Cost Guide
Commercial installations must comply with BS 7671:2018+A4:2026 and the Electricity at Work Regulations 1989. Part P of the Building Regulations does not apply to commercial premises, but the installation must still satisfy the relevant parts of the Building Regulations, particularly Part B (fire safety). These are the regulations that change what a commercial rewire costs.
| Regulation | What it means on a commercial rewire |
|---|---|
| 443.4.1 | SPDs are the default. Omission depends on the owner declaring they are not required. |
| 534.4.4.2 | Protective distance over 10 m — consider additional coordinated SPDs closer to the equipment. |
| 411.3.3 | 30 mA RCD on socket-outlets rated up to 32 A. A documented risk assessment can disapply indent (b) only. |
| 411.3.4 | RCD protection of lighting circuits is a domestic requirement — it does not apply to commercial premises. |
| 421.1.7 | AFDDs are recommended, not required, in offices, retail and industrial units. |
| 560.9 / 560.10 | Emergency lighting to BS 5266-1; fire detection and alarm to the BS 5839 series. |
| 644.1 / 644.4 | An EIC shall be issued on completion by those responsible for design, construction and verification. |
Regulation 443.4.1 requires protection against transient overvoltages where the consequence caused by the 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.
On commercial premises the “significant financial or data loss” limb is usually met outright, and even where it is not, the default position is that an SPD goes in. Price one unless the client puts the declaration in writing. Where SPDs are used they shall be selected and erected in accordance with Section 534. Regulation 534.4.4.2 adds a detail worth carrying into the design: if the protective distance between the SPD and the equipment it protects exceeds 10 metres, oscillations could lead to a voltage at the equipment terminals of up to twice the SPD's voltage protection level, so consider additional coordinated SPDs closer to the equipment or a device with a lower protection level. In a multi-storey office that means budgeting Type 2 units at each sub-distribution board, not just one at the origin.
Regulation 411.3.3 requires additional protection by a 30 mA RCD for socket-outlets rated up to 32 A. It has three indents: (a) socket-outlets in locations where they are liable to be used by persons of capability BA1 or children (BA2), (b) socket-outlets in other locations, and (c) mobile equipment up to 32 A for use outdoors. An exception is permitted to indent (b) — but not to (a) or (c) — where a suitably documented risk assessment undertaken with the involvement of a skilled person (electrically) determines that RCD protection is not necessary. That risk assessment shall be provided with the electrical installation certificate, so allow for producing it rather than assuming a verbal agreement will do.
Note that Regulation 411.3.4, which requires 30 mA RCD protection of final circuits supplying luminaires, applies within domestic (household) premises only. Commercial lighting circuits do not attract it, which is a genuine saving on a large lighting package.
Commercial installations are usually protected by RCBOs on individual circuits rather than bank RCDs, so that a fault on one circuit does not take out several others. Regulation 314.1 backs the approach directly: an installation shall be divided into circuits to (a) avoid danger and minimise inconvenience in the event of a fault and (d) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents not due to a fault. Budget for it at tender — retrofitting RCBOs after a client complains about nuisance tripping is not a variation you will get paid for.
Arc fault detection devices conforming to BS EN 62606 shall be provided for single-phase AC final circuits supplying socket-outlets rated up to 32 A in high rise residential buildings, houses in multiple occupation, purpose-built student accommodation, and care homes. For all other premises — which covers most commercial work — the use of AFDDs is recommended, not required. Where they are used, they shall be placed at the origin of the circuit to be protected. Price them as a priced option and let the client decide, rather than either omitting them silently or loading the tender with them.
Under Regulation 644.1, an Electrical Installation Certificate based on the model in Appendix 6 shall be issued to the person ordering the work on completion of verification. Regulation 644.3 requires it to include the extent of the work covered, Schedule(s) of Inspection, and Schedule(s) of Circuit Details and Test Results — on a large commercial job those schedules run to many pages, which is why testing is not a last-afternoon activity. Regulation 644.4 requires the persons responsible for the design, construction and verification to issue the certificate taking account of their respective responsibilities, and to record the recommended interval between initial verification and the first periodic inspection.
Fire alarm and emergency lighting are certified separately. Regulation 560.9 requires emergency lighting systems to comply with BS 5266-1, BS EN 1838 and BS EN 50172; Regulation 560.10 requires fire detection and fire alarm systems to comply with the relevant parts of the BS 5839 series. Both systems need commissioning certificates and ongoing periodic testing.
08 · Cost Guide
Commercial rewires are high-value contracts with real profit in them and real risk if priced badly. Three things separate the tenders that make money from the ones that do not.
Walk the building and measure every containment route before pricing anything else. It is the biggest variable cost and the one most often guessed. Use Elec-Mate's quoting app to build itemised schedules of containment, distribution equipment and final circuits.
Allow real time for testing and completing the EIC and its schedules. A 500m² commercial installation takes 2 to 3 days to test and certify properly. Add the fire alarm and emergency lighting commissioning certificates, and any documented risk assessment you are relying on under Regulation 411.3.3.
Site supervision, welfare, temporary lighting, tool hire, waste disposal and site access all cost money and none of them appear on a materials list. On a six-week commercial contract, prelims add 8% to 12% to the direct costs.
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