First Fix vs Second Fix Plumbing: What Each Stage Includes

First fix plumbing is the pipework installed before walls are plastered and floors are laid; second fix plumbing is the connection of the visible fittings, such as basins, toilets, radiators and the boiler, once plastering is finished. The difference matters because first fix work stays hidden for decades, and a badly planned pipe run or an untested joint is far easier to correct while the walls are still open.

Lazard Plumbing carries out both stages for homeowners, builders and developers across Havering and Essex, from bathrooms fitted where there was no existing plumbing to work from, to underfloor heating laid in new extensions. If you are planning a renovation or new build, the sign-off checklist below is the one to use before any wall is closed.

First fix vs second fix in one line: first fix runs and tests the hidden pipework before plastering; second fix connects and commissions the visible appliances after it.

What Is First Fix and Second Fix?

First fix and second fix are the two main stages of the services work on a construction project, and plastering is the dividing line between them. First fix covers everything installed inside the structure before walls and ceilings are boarded and plastered, including stud walls, cables and pipework. Second fix covers everything fitted to the finished surfaces afterwards, including doors, skirting boards, switches and bathroom sanitaryware. Each trade on a building project works to this split, so plumbers, the electrician and the carpenter each complete a first fix and return for a second fix.

The terms appear on quotes for new builds, extensions and full bathroom renovations. Replacing a basin has no separate first fix, because the pipework is already in place.

What Is First Fix Plumbing?

First fix plumbing is the installation of all the pipework that will end up hidden inside walls, floors and ceilings, with each pipe left capped in place ready for its fitting. On a typical home or extension, the first fix work includes:

• Hot and cold water supply pipes routed to every tap, shower, toilet cistern and appliance position
• Central heating flow and return pipes to each radiator location
• Underfloor heating pipe loops laid over insulation and run back to the manifold position
• Waste water drainage pipes from baths, showers, basins and sinks, plus the soil connections for toilets
• Gas pipework to the boiler or hob position, which must be done by a Gas Safe registered engineer
• Space, supports and pipe routes for the boiler, hot water cylinder and any water softener
• Fixing points, agreed with the carpenter, for wall-hung toilets, basins and heavy radiators

Pipe routes are the essential decisions at the first fix stage. Pipework through joists has to leave the floor structurally sound, and waste pipes need a continuous fall. Approved Document H sets the sizes and falls for waste pipes, and the right fall depends on the pipe size, its length and how many fittings connect to it. First fix also sets the finished position of every outlet, so bathroom fittings should be chosen before first fix begins.

Why Plumbing Is Tested at First Fix

First fix plumbing is tested before the walls close because every joint is still visible, so a leak can be found and repaired far more easily than by cutting into a finished wall. The Water Supply (Water Fittings) Regulations 1999 require new water systems to be pressure tested before they are commissioned, and Essex & Suffolk Water’s pressure testing guidance explains that the test pressure and method depend on the system and the pipe material. Approved Document H also sets an air test for above-ground waste pipework. Ask for each test result to be recorded.

Underfloor heating loops are pressure tested before the screed goes down and kept filled while it is laid, following the pipe manufacturer’s instructions. A slow leak inside a wall can cause damp and mould, a health hazard as well as a repair bill.

What Is Second Fix Plumbing?

Second fix plumbing is the stage where the plumber returns after plastering to make the final connections, then fill, test and commission every visible fitting on the first fix pipework. Typical second fix tasks include:

• Fitting bathroom sanitaryware: toilets, basins, baths, shower trays and enclosures
• Connecting taps, shower valves, sinks, appliance valves, radiators and towel rails
• Installing the boiler with its flue and condensate pipe, carried out by a Gas Safe registered engineer
• Connecting the hot water cylinder, and the underfloor heating loops to their manifold and thermostats
• Fitting valves that prevent backflow, such as double check valves, where the Water Regulations require them
• Filling, flushing and balancing the system, then commissioning it and handing over the paperwork

Tap heights, radiator positions and shower valve depths were all set at first fix, so the finishing touches at second fix depend on how carefully the first was planned. Unvented hot water cylinders need particular care, because Building Regulations Part G requires them to be installed by a competent person.

What Is Third Fix, or Final Fix?

Third fix, sometimes called final fix, is the last round of fitting and checking once decorating is finished, and not every project uses the term. On Lazard’s construction and development projects, the final fix covers testing the plumbing system for leaks and efficiency, installing taps, showerheads and decorative covers, and handing over a fully functional system to the development team. On smaller jobs, these tasks are simply part of second fix.

First Fix vs Second Fix: The Key Differences

The difference between first fix and second fix is timing and visibility: first fix is the hidden pipework installed before plastering, and second fix is the visible connection work completed after it.

First Fix Plumbing Diagram: What Goes Where in a Bathroom

A first fix plumbing diagram shows each pipe as it is left before plastering: capped at a set height and position, ready for the fitting that connects to it at second fix. The table below works as a written diagram for a typical bathroom.

Ask your plumber to photograph each wall and floor with a tape measure in shot before it is boarded. Those photos become the as-built record and stop anyone drilling a shelf or towel rail fixing into a pipe years later.

How Plumbing Fits With First Fix Carpentry and Electrical Work

First fix plumbing sits in the middle of a set sequence: first fix carpentry creates the structure, the plumber and electrician run their services, and then the walls are boarded and plastered. A typical build process runs in this order:

• Structure and shell: foundations, walls and roof complete, so the building is weathertight.
• First fix carpentry: stud walls, floor joists, door frames, door linings and staircases.
• First fix plumbing and first fix electrical: pipework, cables and back boxes. Plumbing usually goes first, because waste pipes and their falls are harder to reroute than cables.
• Testing and inspection before anything is covered.
• Plasterboard, plastering and screeding, followed by drying time.
• Second fix carpentry: fitting doors, skirting boards, architraves and kitchen and bathroom units.
• Second fix plumbing and electrical installation: sanitaryware, radiators and appliances, plus electrical fixtures such as socket fronts, face plates, switches and light fittings.
• Decoration, final fix, testing and handover.

Clashes with other trades happen where they share space, such as a waste pipe and a downlight competing for one joist bay. On a larger building project, a project manager agrees these routes before anyone drills, and on a timber frame home the frame manufacturer’s guidance applies before any member is drilled or notched.

What Hidden Faults Look Like When Finishes Come Off

Hidden faults usually show up years later as leaks, damp or failed fittings, because they sit behind finished surfaces where nobody can see them. A Lazard project in Hornchurch shows how this plays out. A homeowner asked for a cracked, leaking shower tray to be replaced. Once the tray was lifted, the Lazard team found that it had not been adequately supported and showed signs of insufficient waterproofing, and several sections of the timber subfloor had deteriorated from prolonged moisture. 

The job became a structural floor repair: damaged flooring removed, rotten timber replaced, and the shower base rebuilt level and prepared for waterproofing before the new tray went in. The cause sat under the finish, not in the visible tray.

The same principle applies to first fix plumbing: the work you cannot see after plastering decides how long the visible finish lasts. Untested joints, waste pipes without enough fall, pipes run where fixings will later go and fitting positions changed after first fix are all easier to prevent before plastering than to repair afterwards.

First Fix Sign-Off Checklist Before the Walls Are Closed

A first fix sign-off checklist confirms that every hidden pipe is in the right place and tested before plasterboard goes up or screed is poured. Work through these checks with your plumber:

• Confirm final choices for every fitting: toilet, basin, shower valve, bath, radiators and towel rails, so outlet positions match.
• Check heights and positions against the drawings: tap heights, shower valve depth and radiator bracket positions.
• Ask for the pressure test record for supply and heating pipework, and the air test result for waste pipes.
• Check the fall on every waste run, especially long runs to the soil stack.
• Photograph every wall and floor with a tape measure in shot, and keep the photos with the house documents.
• Confirm fixing points are in place for wall-hung toilets, basins, radiators and heavy towel rails.
• Make sure isolation valves and the underfloor heating manifold will stay accessible once finishes go on.
• Confirm underfloor heating loops are pressure tested and kept filled while screed is laid.
• Check space, flue position and condensate route for the boiler, and the discharge route for any unvented cylinder.
• Check whether any of the work needs notifying to the water company before it starts (see the regulations section below).

How Long Do First and Second Fix Plumbing Take?

First fix and second fix plumbing each take anything from a day to several weeks, depending on the size of the project and the number of rooms involved. A single bathroom first fix is far shorter than the first fix for a whole new build, and second fix is usually quicker than first fix because the routes are already set. The gap between the two is set by plastering, drying and tiling, and after second fix the remaining time to completion is mainly commissioning, snagging and decoration. Our guide to how long a bathroom refurb takes breaks this down for a single room.

Good planning shortens the first fix stage. On an underfloor heating project in Hornchurch for one of Lazard’s long-standing building partners, the client did not want to chase into the existing extension floor. The team fitted 20mm insulating panels in one day instead, and the tiles were laid straight over them the next day.

What Affects the Cost of First and Second Fix Plumbing?

The cost of first and second fix plumbing depends on the scope of the job rather than a standard rate, which is why Lazard prices each project with a clear written quotation after assessing the work. The main factors are:

• The number of bathrooms, kitchens, radiators and appliance positions
• Whether there is existing pipework to work from: on one Upminster bathroom Lazard installed, there was no original plumbing at all
• The heating type: radiators, underfloor heating or both, and whether a new boiler or unvented cylinder is included
• Access: floor voids, joist direction, solid floors and timber frame restrictions
• Pipe materials and fitting choices
• How the work is phased around other trades, since each return visit is part of the plan

The quotation sets out all the work included, plus important exclusions and assumptions. Any issue discovered during the work is discussed with you before extra work goes ahead.

Which Regulations and Certificates Apply?

First and second fix plumbing in England is governed by the Water Supply (Water Fittings) Regulations 1999, Building Regulations Parts G and H, and gas safety law:

• Water Regulations: Regulation 5 requires notice to the water company before certain work starts, including erecting a new building, installing a bath holding more than 230 litres, or fitting a pump drawing more than 12 litres per minute.
• Gas: the HSE states that anyone employed to work on gas appliances in domestic premises must be a Gas Safe registered engineer.
• Hot water and drainage: Approved Document G covers unvented hot water systems, which must be installed by a competent person, and Part H sets waste pipe sizes, falls and air testing.

First and second fix plumbing is work for a qualified plumber, not DIY. Gas work must be done by a Gas Safe registered engineer, unvented cylinders by a competent installer, and notifiable work needs water company consent before it starts; the homeowner’s role is to agree positions, check the test records and ask questions before the walls are closed.

Planning a Project With First and Second Fix Plumbing?

The best time to bring in a plumber is before first fix carpentry starts, while pipe routes, fitting positions and heating choices can still be planned around the structure. Lazard has served more than 6,000 customers across Upminster, Hornchurch, Romford, Brentwood, Billericay, Basildon and Havering, has over 1,000 genuine reviews on Google and Checkatrade, and uses Gas Safe registered engineers for gas work alongside CIPHE membership. 

Completed work is backed by a 24-month workmanship warranty, with manufacturer warranties applying separately to products, and a dedicated office team keeps you updated from survey to handover.

If you are planning a bathroom renovation, extension or development, get a quote for a survey and written quotation, or ask a question about your project first.