Where should the boiler go in an extension?

On or near an external wall, because the flue has to vent combustion gases outside with strict minimum clearances from windows, boundaries, and roof lights under Approved Document J. Every pipe in the extension runs to or from the boiler, so its position is the first decision that constrains all the others.

Do you need a combi or a system boiler for an extension?

A combi is usually fine for a single-storey extension that does not add a bathroom, but it struggles when two showers and a tap run at once. A system boiler with an unvented cylinder gives consistent pressure across multiple outlets and suits three or more bathrooms or high simultaneous demand.

How far can a sink be from the soil stack before you need an air admittance valve?

Once a waste run gets long, the trap seal can be siphoned out, causing gurgling and drain smells, so an air admittance valve is fitted at the high point of the run to prevent it. Your plumber sizes this against the run length and pipe diameter set out in Approved Document H.

What waste pipe gradient does an extension need?

Waste pipes must fall towards the drain within the range Approved Document H sets for the pipe size. Too shallow and waste sits in the pipe; too steep and water outruns the solids, leaving them to build up and block. Both extremes fail.

Can you mix push-fit plumbing brands like Speedfit and Hep2O?

No. The pipe inserts are not interchangeable, and putting one brand's insert into the other's fitting is a common cause of leaking joints. Pick one brand, tell your plumber, and stick with it throughout the installation.

Back to Single Storey Rear Extension

Phase 6 · First Fix · Task 02 of 06

Plumbing Layout Planning

How to plan your water supply, heating, and waste pipe positions before the plumber arrives. Copper vs push-fit, boiler positioning, and the decisions that can't wait.

16 min readUpdated 2026Premium

Process infographic showing the plumbing layout planning sequence: finalise room layout, choose boiler position, map supply and waste routes, coordinate with electrician, then brief the plumber

Prerequisites

Complete this first

Electrical Layout Planning

A plumber who doesn't know where your sink goes will put pipes where it suits the pipe run, not where it suits your layout. Plumbers routinely chase pipework into the wrong wall during first fix when nobody provides a layout drawing. They assume the sink is on the wall closest to the existing plumbing. Reasonable assumption. Wrong wall. That's rerouted pipes, wasted time, and walls that need making good before they've even been plastered.

This is the planning stage. The actual installation (first-fix plumbing) is a separate task that happens after you've completed this exercise. You are the designer. Your plumber executes your design. If you hand them a phone call instead of a drawing, you'll get assumptions instead of answers.

Do this first

Your room layout must be finalised before you plan plumbing positions. The sink position determines supply and waste pipe routes. Appliance positions determine branch connections. The boiler location determines everything else. If the layout changes after first-fix pipes are buried in walls and screed, rework is expensive and disruptive. Don't start this until your layout drawings are complete.

What this guide covers

01Boiler Position: The Decision That Drives Everything
02Copper vs Push-Fit: The Material Decision
03Pipe Sizing
04Waste Pipe Routing
05Appliance Connections
06Underfloor Heating Coordination
07Sequencing and Coordination
08Building Regulations
09Creating Your Plumbing Layout Drawing
10What Your Plumbing Quote Should Include
11Common Mistakes
12What This Costs You to Plan

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