Not every property needs a full rewire. A partial rewire targets only the defective circuits and saves 40-60% of the cost. This guide explains when a partial rewire is the right choice, how to connect old and new wiring safely, and what certification is required.
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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.
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A partial rewire is the replacement of specific sections of the electrical wiring in a property, rather than the entire installation. It targets the circuits, cables, or areas that are defective, dangerous, or no longer adequate — while leaving the satisfactory sections of wiring in place.
This approach is common in properties where the wiring is a mix of different ages. For example, a 1970s house that had a kitchen extension added in 2005 might have original PVC wiring throughout the main house but modern twin-and-earth cable in the extension. If the original wiring in the main house is deteriorating but the extension wiring is fine, a partial rewire of the main house circuits makes more sense than a full rewire.
A partial rewire can include replacing individual circuits (for example, all lighting circuits), replacing wiring in specific areas (for example, the ground floor only), or upgrading the consumer unit and main distribution while retaining the final circuits.
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The decision to carry out a partial rewire rather than a full rewire should be based on the findings of an EICR. The report will identify which circuits have defects and which are satisfactory. A partial rewire is appropriate when:
The cost savings from a partial rewire come from reduced labour (fewer circuits to install and terminate), reduced materials (fewer cables, accessories, and back boxes), and less making good (fewer walls to chase, fewer floorboards to lift). The consumer unit is often the largest single cost item in any rewire — if the existing consumer unit is being replaced regardless, the marginal cost of connecting additional circuits is relatively small, which shifts the calculation towards a fuller scope of work.
Elec-Mate's AI Cost Engineer analyses the EICR defects, calculates materials and labour for each circuit, and generates a detailed quote.
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The technical challenge of a partial rewire is the interface between old and new wiring. Where new cables connect to existing circuits or where new circuits share a consumer unit with existing ones, the electrician must ensure:
A common approach is to install a new consumer unit and reconnect all circuits (both new and existing) to it. This ensures all circuits have RCD protection and modern MCBs or RCBOs, even if the final circuit cables are a mix of ages.
A partial rewire requires full inspection and testing of the new work, and verification testing of the existing circuits that connect to or are affected by the new work. The testing sequence follows the standard order set out in BS 7671 and GN3:
The electrician should also carry out a visual inspection of the existing circuits to confirm they have not been damaged during the partial rewire work. If the consumer unit has been replaced, all circuits (new and existing) should be tested from the new board.
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Start free trialA partial rewire requires an Electrical Installation Certificate (EIC) for the new work. The EIC must include a schedule of inspections and a schedule of test results covering all new circuits and any existing circuits that were altered or reconnected.
If the partial rewire includes new circuits (not just replacement of existing cables on existing circuits), the work is notifiable under Part P of the Building Regulations (England and Wales). The electrician must either self-certify through a competent person scheme or notify building control directly.
A consumer unit replacement as part of a partial rewire is also notifiable. The EIC should cover the consumer unit and all circuits connected to it, including existing circuits that were reconnected.
Important: The EIC for a partial rewire should clearly describe the extent of the work — which circuits were replaced, which were retained, and where the connection points between old and new wiring are located. This information is essential for future inspections.
The most common partial rewire scenario. The ground floor circuits (ring main, kitchen circuit, lighting) are replaced with new cables run under the first-floor floorboards. Upstairs circuits are retained if in satisfactory condition. Consumer unit is typically replaced to provide RCD protection for all circuits.
Older properties often have lighting circuits wired in 1.0mm cable with no CPC (circuit protective conductor) — a common C2 defect. Replacing only the lighting circuits with modern 1.5mm T&E cable (with CPC) addresses the defect while leaving satisfactory power circuits in place.
Upgrading an old fuse box to a modern consumer unit with RCBOs, plus replacing the main tails and meter tails if needed. This provides RCD protection for all existing circuits without rewiring the final circuits. Often the most cost-effective improvement.
Kitchens and bathrooms are special locations under BS 7671 and have specific requirements for circuit protection and IP ratings. Rewiring just the kitchen or bathroom circuits to meet current standards is a common partial rewire scope, particularly after a kitchen or bathroom refurbishment.
Partial rewires require careful scoping. Underquote and you lose money; overquote and you lose the job to someone recommending a full rewire. The key is a thorough initial survey with accurate defect identification.
Start with an EICR. Every C1 and C2 defect identifies a circuit or section that needs work. Elec-Mate's remedial works estimator prices each defect — replacing a circuit, upgrading a consumer unit, adding RCD protection — and generates a detailed quote.
The AI Cost Engineer calculates exact material quantities (cable metres, back boxes, accessories, consumer unit specification) and labour hours based on the scope of work. It uses live trade pricing data and adjusts for your region.
When the partial rewire is complete, produce the EIC on site with voice test entry, generate the invoice, and send everything to the client before leaving. No desk time, no chasing.
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