What does first fix plumbing cover on an extension?

It is all the buried pipework: hot and cold supply runs to every tap and appliance, waste pipes to the soil stack, gas pipework to the boiler, and any heating manifold connections. Everything goes in before plaster and screed seal it inside the walls and floor.

Is push-fit or copper better for first fix plumbing?

Push-fit plastic is standard for most hidden supply runs because it is fast, flexible, and corrosion-resistant, and it is fully building-regs compliant. Copper is still used for gas, the boiler connection, and visible pipework. The leak risk with push-fit comes from poor installation, not the material itself.

Why must first fix plumbing be pressure-tested before walls close?

Once plasterboard and screed cover the pipes you cannot see a leak until it has already caused damage. A pressure test while the joints are still exposed proves the system is sound, and building control will expect evidence the test was done.

How long does first fix plumbing take on an extension?

Most extensions take a plumber one or two days for first fix. The pipework itself is quick; the time and risk sit in getting the right pipes into the right walls and confirming the pressure test before anything is covered up.

Who is responsible for the Gas Safe certificate after first fix?

The Gas Safe registered engineer who did the work must notify the local authority within 30 days, and the compliance certificate is then posted to you. It is their legal duty, not yours, but you are the one blocked from building control sign-off if it never arrives, so withhold final payment until it does.

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Phase 6 · First Fix · Task 04 of 06

First Fix Plumbing: Pipes and Connections

What first fix plumbing covers in a UK extension, costs (£4,000 – £7,000), and the communication failures that turn a simple job into a 14-month paperwork nightmare.

11 min readUpdated 2026Premium

Process infographic showing the first fix plumbing workflow from layout plan through pipe installation, pressure testing, and sign-off before walls are closed

Prerequisites

Complete these first

Your plumber will probably be on site for one or two days. The pipework itself is the easy bit. What's hard is making sure the right pipes end up in the right walls, that nobody skips the pressure test, and that the Gas Safe certificate actually arrives. One build waited 14 months for that certificate, and building control couldn't sign off the extension until it turned up.

First fix plumbing covers everything that gets buried: hot and cold water supply pipes running from the mains and boiler to every tap and appliance, waste pipes carrying dirty water from sinks and appliances to the soil stack or drain, and gas pipework from the meter to the boiler (and to the hob, if you're running gas). Once the plasterer and screeder have finished, this pipework is sealed inside your walls and floors for the life of the building. Getting it wrong means tearing plaster off walls to fix it.

What this guide covers

01What gets installed
02Push-fit vs copper
03Wall chases and structural limits
04What it costs
05What to check before walls close
06The Gas Safe certificate problem
07Coordinating with other trades
08Cold water and insulation requirements

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