Boiler, Consumer Unit and Drains: Will Your Services Cope?

Every extension plugs into three installations that were sized for a smaller house: the heating system, the electrical supply and the drains. Building Control will check that the new work is compliant. Nobody in the process is asked whether the old equipment has anything left in it, and no quotation contains a line for finding out.

That gap is where a surprising amount of money goes. A cold room at the far end of a warm house, a shower that dies when the dishwasher fills, a consumer unit that cannot take another circuit, a soakaway that was never sized for the roof you have just added. Each of them is cheap to design around, moderately priced to fix while the floor is open, and expensive afterwards. What the regulations require of the new work is a separate question from this one, and answering it does not answer this.

The order these questions get asked in

All of them belong at survey stage, because each has an answer that changes the drawing.

Drainage is settled first, because it decides where a sink, a WC or a utility can go at all, and on a solid floor it goes in the ground before the concrete. Heating comes next, since it decides how much wall the room loses and whether the plant is staying. Electrics can wait a little longer, though not much, because the route back to the board is easier while there is no wall in front of it. None of the three is hard to establish: a heat loss calculation, a bucket and a watch, a screwdriver, and an afternoon lifting covers.

What the extension actually adds to the heating

The load is not floor area, it is heat loss, and two rooms of the same size can differ by a factor of two.

A new extension is built to current insulation standards, so per square metre it loses far less than the house it is attached to. What it does have is glass. A twenty square metre room with a modest window and a well detailed flat roof may add less to the system than the owner fears. The same room with a full width glazed screen, a lantern and three rooflights, is a different proposition, because glazing loses several times what an insulated wall loses over the same area.

The right way to find out is a room by room heat loss calculation for the house as it will be, which is a standard piece of design work and not an expensive one. It produces two numbers you need: the total load against the boiler’s output, and the load in each room against the emitter you are proposing to put in it.

Output is rarely the binding constraint. Most domestic boilers are sized by hot water demand rather than by space heating, and a combination boiler bought to run two bathrooms typically has far more heating capacity than the house has ever needed. The symptom worth taking seriously is not a cold extension in January. It is a boiler that already fails to reach temperature on the coldest week, or that fires and stops in short bursts, or a house whose upstairs goes cold whenever a bath is running. Those say the existing system is at a limit before you add anything, and they are worth diagnosing rather than absorbing.

The radiators are the part that catches people

A radiator’s stated output is quoted at a temperature difference, and if the system does not run at that temperature the number on the box is fiction.

Heat output falls steeply as flow temperature drops. A radiator producing its catalogue figure with water arriving at seventy five degrees gives roughly half of it at fifty five. So a new radiator sized from a catalogue and fitted to a system running cooler than the catalogue assumed will not heat the room, and the usual conclusion, that the boiler cannot cope, is the wrong one.

There is a Part L requirement in this area and its scope is narrower than it is usually reported. Where a wet heating system is newly installed, or fully replaced in an existing building including the appliance, the emitters and the pipework, the whole system should be sized to work at a maximum flow temperature of fifty five degrees or lower. Extending an existing system into a new room is neither of those things, and neither is swapping a boiler on its own. Building an extension does not oblige you to redesign the heating in the rest of the house.

It is still the number to design to voluntarily, for one reason. If a heat pump ever replaces the boiler, everything has to work at a low flow temperature, and the emitters are what stops it. An extension fitted out with radiators sized generously, or with underfloor heating in a screed, is already on the right side of that. One sized tight at seventy five degrees is the room that will be cold in fifteen years.

Two practical points come with it. The extension is usually the furthest point on the circuit, so pipe sizing and pump duty matter as much as radiator size. And a large glazed room open to the rest of the ground floor has a completely different heat loss profile from a small front room, so putting the two on one thermostat produces one space that is too warm and another that is too cold.

If the boiler is being replaced anyway

A boiler swap done during the build costs more than a boiler swap done on its own, and it is still the cheapest week in ten years to do it.

Replacing the appliance brings requirements the extension itself does not. A replacement gas boiler has a minimum seasonal efficiency of ninety two per cent. A gas combination boiler installed in an existing dwelling must come with at least one additional efficiency measure: flue gas heat recovery, weather compensation, load compensation, or a smart thermostat with automation and optimisation. And where the heat generator is replaced, each room should be given thermostatic room control, which in most houses means valves on the radiators that do not have them and a proper room thermostat rather than a dial in the hall.

Hot water is a different question from heat

A new bathroom is a demand problem, and the ceiling on it is usually the incoming main rather than the boiler.

A combination boiler heats water as it passes through, so its hot water performance is a flow rate at a temperature rise, and it cannot exceed what the mains delivers. Measure that before anything is specified: run the cold tap at the kitchen sink with everything else off, fill a container of known volume and time it, and note the static pressure. An older property fed by a small bore iron or lead main can be well under what a modern combination boiler assumes, and no appliance will improve on it.

Then think about simultaneity. A combination boiler serves one outlet properly at a time, so an extension containing a second shower room changes the question from how hot to how many at once. An unvented cylinder answers that by storing hot water at mains pressure, but it needs adequate incoming pressure and flow to be worth having, it occupies a cupboard, and it is a controlled service in its own right with requirements on its safety devices and on where the discharge pipe terminates. It is installed and certified by somebody competent to do that work, and the paperwork belongs with the completion certificate. Putting a bathroom in an extension covers the rest of that job.

The consumer unit, and the word to ask about is ways

The question is not whether the board is old. It is whether it has spare ways and whether the new circuits can be protected properly.

A kitchen extension typically wants more circuits than owners expect: sockets, a dedicated supply for an induction hob or an oven, lighting, extract, sometimes a heating manifold and an outside socket, with a car charger a plausible addition later. Open the board, count the empty positions, and photograph the label.

Adding circuits does not automatically require a new board. What forces one is the absence of spare ways, or an old unit for which no compatible protective device is still available, or a board in poor condition. New circuits need thirty milliamp residual current protection, and cables buried less than fifty millimetres deep in a wall need it too, so on an older board with no such protection the practical answer is often a replacement rather than an addition. Consumer units in houses have been required to have a non-combustible enclosure since the beginning of 2016, so an existing plastic one is not unlawful, but a new one will be steel. Your electrician may also propose arc fault detection, which for an ordinary house is a recommendation rather than a requirement.

Two things ride along with a board change. The main protective bonding to the incoming gas and water services gets inspected and is very often found undersized on older installations, which is a modest cost done at the same time and an awkward one done separately. And the incoming supply fuse, usually sixty, eighty or one hundred amps, is a ceiling on everything. Add an induction hob, an electric shower and a car charger to a house on a sixty amp cut-out and the limit becomes real. Upgrading the cut-out is the network operator’s work, not your electrician’s, and it runs on its own timetable.

All of this is notifiable. New circuits, a replacement consumer unit and any work in a room containing a bath or shower have to be certified, either self certified by an electrician on a competent person scheme or notified to Building Control with the rest of the job. You should finish with an electrical installation certificate to keep alongside the completion certificate.

Drains, where the pipe is almost never the problem

Foul capacity is rarely the constraint. Fall, route and what sits above the pipe are.

A drain carrying the discharge from a WC is at least one hundred millimetres in diameter, and laid at one in forty it will carry around nine litres a second. The whole appliance group in a family house does not come close to that even when everything runs at once. Adding a bathroom to an extension does not overload a foul drain.

What does bite is geometry. Waste needs a fall to reach the existing system, and every metre the new sink or WC sits further from the stack costs another twenty five millimetres of drop at that gradient. Over twelve metres that is three hundred millimetres, and the invert still has to arrive above the level of the pipe it is joining. On a solid floor this is designed before the concrete goes in, and moving a sink after the slab is down is an expensive change of mind. Where gravity genuinely does not work, a pumped system is the answer and it is better chosen than discovered.

Then there is what the extension sits on top of. A manhole inside the new footprint is either relocated or fitted with a sealed double sealed cover, and you may not build over an access point on a run serving more than one property without providing an acceptable alternative. Building over or within three metres of the centreline of anything shown on the sewer records brings in a separate consent from the water company, on its own timescale, and the guidance carries specific triggers: more than six metres of pipe under the building, a run deeper than three metres, or a diameter over two hundred and twenty five millimetres, each needing the owner’s agreement. When a build over agreement is needed sets out the tests.

Surface water is the capacity question people miss

The new roof has to go somewhere, and the existing arrangement was sized without it.

Rainwater should discharge to a soakaway or another infiltration system where that is practicable, then to a watercourse, which may need consent and a limited discharge rate, and only where neither is workable to a sewer. Adding twenty five or thirty square metres of roof to a plot drained by a soakaway means that soakaway is now undersized, and the way that shows is a wet patch in the lawn every autumn rather than a dramatic failure.

Three constraints govern the fix. An infiltration device should not sit within five metres of a building or a road, which on a small plot is most of the garden. It is sized from a percolation test and designed for a storm with a ten year return period, so the ground has to be tested rather than assumed, and stiff clay frequently fails that test outright. And where the plot drains to a combined sewer, adding surface water to it is something the water company allows only where the capacity exists.

What to check before anything is drawn

1. A room by room heat loss calculation for the house as extended, with the glazing you actually want in it.
2. The flow rate and static pressure at the kitchen tap, measured rather than estimated.
3. The consumer unit opened, the spare ways counted and the label photographed.
4. The incoming supply fuse rating, which is printed on the cut-out.
5. Every cover on the plot lifted, the falls followed and the depths noted.
6. Where the rainwater goes now, which is often nobody’s idea and simply what the last owner did.

None of that costs more than an afternoon, and between them they settle whether the plan on the table is buildable as drawn. We work through it at survey, before anything is priced, because the answers move walls. Send us the address and what you are hoping to build.

The requirements described here are revised from time to time and their application depends on your property and the work proposed. Treat this as an explanation of how the assessment is made rather than as a specification for your house.

Last reviewed August 2026. Planning and Building Regulations change, and your property may differ.