Energy Supply and Capacity When Planning a House Extension or Small Commercial Build
Enhancing your home or building a small commercial space? Don't let modern energy demands exceed your current supply; plan ahead!
Most domestic properties in the UK have a single phase electricity supply rated between 60 and 100 amps, which was perfectly adequate when the house was built but may fall short once you add modern high demand equipment. If your extension or outbuilding will include a heat pump, electric vehicle charger, induction hob or air conditioning, you need to assess your supply capacity early in the design stage. Getting this wrong can mean delays, unexpected costs or electrical installations that cannot perform as intended.
Understanding Your Existing Supply
Your electricity supply has a maximum capacity, set by your Distribution Network Operator rather than the company you pay your bills to. The DNO owns and maintains the cables, substations and meters in your area. There are six DNO regions across Great Britain, including UK Power Networks in the South East and East, National Grid Electricity Distribution in the Midlands and South West, and Scottish Power Energy Networks in Central and Southern Scotland.
A typical domestic property has a main fuse rated at 60, 80 or 100 amps. You can usually find this rating on the main fuse near your meter. This figure represents the maximum current your supply can safely deliver at any one time.
To understand whether your current supply is sufficient, your electrician will need to carry out a maximum demand calculation. This assessment adds up the expected load from all circuits, applying diversity factors set out in BS 7671, the UK wiring regulations. Diversity accounts for the fact that not everything runs at full power simultaneously, but even with these allowances, modern all electric homes can push close to or beyond the limit of a standard supply.
High Demand Equipment to Consider
Several types of equipment commonly specified in extensions and new outbuildings place significant demands on electrical supplies:
- Air source and ground source heat pumps typically draw between 3 and 12 kW depending on the system and property size
- Electric vehicle chargers for home use are commonly rated at 7 kW, though faster units may require more
- Induction hobs can draw 7 to 11 kW at peak load
- Electric showers usually require 8 to 10 kW
- Air conditioning units vary widely but larger split systems may need 3 to 5 kW per unit
- Commercial catering equipment, server rooms or workshop machinery can have substantial requirements
If you are planning to install more than one of these items, or if your existing property already has high electrical demand, your supply may need upgrading before or during the build.
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If your electrician's calculations show that your existing supply is inadequate, you will need to apply to your DNO for an upgrade. This is a separate process from anything your energy supplier handles.
The typical steps are:
- Contact your DNO to request a quotation for the work required
- Provide details of your property, current supply and proposed additional load
- Receive a formal quotation, which will outline the scope of work, costs and estimated timescale
- Accept and pay for the work, then wait for the DNO to schedule the installation
Timescales vary considerably. A straightforward fuse upgrade from 60 to 100 amps on an existing single phase supply may take four to eight weeks. More complex work, such as installing a new three phase supply or upgrading cables from the street, can take several months and may involve significant groundworks.
Costs depend on the extent of work required. A simple fuse upgrade may cost a few hundred pounds, while bringing in a new three phase supply or reinforcing the local network can run into thousands. Your DNO must provide a clear breakdown of charges in their quotation. You have the right to raise a complaint with the Energy Ombudsman if the quote seems unreasonable, and Ofgem publishes guidance on connection charging.
Your situation may be slightly different. ask a question below ↓ and our editorial team will reply with our advice.
Single Phase Versus Three Phase Supply
Most homes have single phase electricity, delivered through one live conductor. Three phase supplies use three live conductors and are standard for commercial and industrial premises. They allow higher total capacity and are better suited to large motors, commercial kitchens or properties with very high electrical demand.
For domestic extensions, a three phase supply is rarely necessary unless you are installing substantial workshop equipment or multiple high demand systems. However, for small commercial builds, light industrial units or large properties with heat pumps, EV charging and extensive electrical loads, three phase may be the practical choice.
Three phase installations cost more upfront and require different distribution boards and wiring. Your electrician can advise whether the additional complexity is justified for your project.
Gas Connections, EV Charging and Design Stage Planning
If you are building a garden room, detached home office or annexe, you may wonder whether to bring gas to the new structure. In many cases, an all electric approach using efficient heating and good insulation will be simpler and more cost effective. Gas connections to outbuildings involve trenching, meter installation and compliance with Gas Safe regulations, all of which add expense and disruption.
However, there are situations where gas may still make sense. If the main house has a gas supply and the outbuilding will have high heating demand, or if you are building a commercial kitchen where gas cooking is preferred, the economics may favour a gas connection. Your architect or building services engineer can model the options.
Bear in mind that building regulations are increasingly favouring low carbon heating. From 2025, new homes in England must meet the Future Homes Standard, and similar rules are expected in Wales. Scotland has its own building standards with strong energy efficiency requirements. These new standards could impact your decision on gas usage.
Building regulations in England now require new homes and buildings undergoing major renovation to include EV charge point infrastructure. Even where not strictly required, installing charging provision during a build is far cheaper than retrofitting later.
At minimum, consider running appropriate cabling and installing a suitable distribution board way for a future charger. If you plan to install a charger immediately, ensure your maximum demand calculation accounts for it. A dedicated circuit is required, and the installation must comply with BS 7671 and relevant product standards.
If you are installing multiple chargers, for example in a small commercial car park, load management systems can help balance demand across units and reduce the required supply capacity.
Addressing energy supply early avoids costly surprises. When briefing your architect, designer or electrician, consider asking:
- What is the rating of the existing main fuse and is an upgrade likely to be needed?
- Has a maximum demand calculation been carried out for the completed project?
- Will the DNO need to be contacted, and if so, who is handling this?
- Is three phase supply worth considering for our requirements?
- How will EV charging provision be incorporated?
- Are there any local network constraints that might affect connection timescales?
Your electrician should be registered with a competent person scheme such as NICEIC, NAPIT or ELECSA. This ensures their work is compliant with Part P of the building regulations and BS 7671, and that certificates are properly issued and registered.
Frequently Asked Questions
How do I find out who my DNO is?
You can identify your DNO by entering your postcode on the Energy Networks Association website. Your DNO is different from your energy supplier and is responsible for the physical network infrastructure.
Can my electrician upgrade the main fuse?
No. The main fuse belongs to the DNO and only they or their appointed contractors can replace or upgrade it. Your electrician can carry out work on your side of the meter.
Will a supply upgrade delay my build?
It may do. Straightforward upgrades can be completed in weeks, but complex work involving new cables or network reinforcement can take several months. Apply as early as possible once your requirements are known.
Where can I find more information on connection charges?
Ofgem publishes guidance on electricity connection charges and your rights as a customer. Your DNO must also provide a clear quotation and explanation of costs before work begins.
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