Plumbing Budget Calculator

Plumbing for a UK kitchen extension usually costs £8,000 – £18,000 once boiler work is part of the job. Take the boiler out and the first fix plumbing on its own, the buried supply and waste pipework, is typically £4,000 – £7,000. This calculator turns those broad ranges into a figure built around your actual scope: the boiler decision, whether you are laying underfloor heating, how many bathrooms are served, and each outlet you want plumbed.

The estimate splits into two stages, because that is how a plumber quotes and invoices. First fix is the pipework buried in walls and floors before plastering. Second fix is the visible work at the end: taps, waste traps, appliance connections, and commissioning the boiler. Add-ons like a boiling water tap sit outside both stages and are usually priced on their own.

Where the money goes

The boiler is the biggest single variable. A new combi boiler, supplied and installed by a Gas Safe registered engineer, is usually £2,000 – £3,500 at second fix, on top of the first-fix gas service and roughing-in. Relocating an existing boiler instead costs £500 – £2,800, driven by distance and whether gas and water runs have to be extended. If no boiler work is needed at all, this whole line drops away and the estimate falls sharply.

Beyond the boiler, most of the cost is the plumber's labour running supply and waste to each point. Pipe and fittings themselves are cheap. Adding underfloor heating brings a per square metre charge for the pipe loops and manifold connection, which is first-fix work that has to go in before the screed is poured. Each bathroom on the scope adds its own supply and waste rough-in at first fix and fixture connections at second fix.

The add-ons that quietly grow the bill

The line items that catch people out are the ones a standard quote leaves off. A boiling water tap runs £1,400 – £2,900 for a basic model and up to £3,500 – £5,500 for a version with a chilled and sparkling unit. A water softener, unit plus professional installation, is around £475 – £2,150. An outside tap is £80 – £150, and it needs a double check valve under the Water Supply (Water Fittings) Regulations. None of these are usually inside the main plumbing contract, so specify everything upfront and get written confirmation of what is and is not included before work starts.

A worked example

Take a 30m² kitchen extension with a new combi boiler, wet underfloor heating across the open-plan floor, a kitchen sink and dishwasher plumbed in, and a boiling water tap.

The new combi drives the largest lines: the second-fix supply and installation at £2,000 – £3,500, plus its first-fix gas service and pipework. The underfloor heating adds a per square metre charge for loops and manifold across the heated area. The sink and dishwasher each add a supply run and waste connection at first fix and their final connections at second fix. The boiling water tap is a separate add-on at £1,400 – £2,900 for a basic unit, more if you choose the chilled and sparkling model.

The calculator folds these into the first fix and second fix totals shown above and gives a headline plumbing budget. For a project with boiler work, that headline lands inside the £8,000 – £18,000 full-scope range. A documented real kitchen extension with a new boiler, underfloor heating, a softener, and a boiling water tap came in near the lower half of that range, so treat the middle of the range as a sensible planning figure and hold a buffer for the add-ons that appear late.

One last point on the boiler: gas work must be certified for building control sign-off, and the engineer has to notify the Gas Safe Register within a set window of finishing the installation. On one real project the paperwork, not the pipework, held up the completion certificate for over a year because the engineer never filed the notification. Chase that certificate from day one and withhold the final payment until you have written proof it has been done.