First Fix vs Second Fix: What These Construction Terms Mean for Your London Project

If you have ever received a construction quote or discussed a project with a contractor, you have almost certainly heard the terms "first fix" and "second fix." These are fundamental to how construction projects are programmed and priced in London. This guide explains what each term means across the main trades.

The Basic Principle

Construction works are divided into two broad phases relative to the plastering and internal finishing works:

First fix: All work that happens before the walls are plastered (or before final surface finishes are applied). This includes pipe runs, cable routes, structural frames, and anything that needs to be concealed within or behind the finished surfaces.

Second fix: All work that happens after the walls are plastered and surfaces are finished. This includes connecting, fitting, and commissioning all the visible elements — light fittings, sockets, taps, sanitaryware, kitchen units, doors, and so on.

First Fix by Trade

Carpentry (first fix):

• Structural timber work: roof joists, floor joists, stud partitions, lintels
• Roof structure and sarking boards
• Window and door frames (the frame is first fix; the door itself and furniture are second fix)
• Staircase structure (carcase)
• Noggings and blocking for future fittings (towel rails, kitchen units, wall-hung sanitaryware)

Electrics (first fix):

• Consumer unit installation (new or upgraded)
• All cable routing and chasing
• Back boxes for sockets and switches (the metal or plastic box in the wall)
• Lighting positions (cables to ceiling lighting points)
• Loft void, underfloor, and ceiling void cable runs
• Data and TV cable distribution

Plumbing (first fix):

• All pipework runs — hot and cold water supply
• Soil stack installation
• Waste pipework
• Underfloor heating pipe installation (for wet systems)
• Heating pipe distribution
• Boiler installation (boiler is sometimes split — structural connections in first fix, commissioning in second)

The Critical Middle Phase: Plastering and Finishes

Between first and second fix comes the internal finishing phase:

• Plasterboarding all new partitions and ceilings
• Skimming (wet plaster coat over board or existing surfaces)
• Drying out period — plaster must be fully dry before second fix and decoration begin
• Flooring installation (in most programmes, at least the screed or base layer)

This phase typically takes three to five weeks in a standard single-storey extension or refurbishment. Rushing it — moving to second fix before plaster is dry — causes paint failure, joint movement, and cracking.

Second Fix by Trade

Carpentry (second fix):

• Internal doors, frames, and liners
• Skirting boards, architraves, window boards
• Staircase balustrade and handrail
• Kitchen unit installation (the cabinet installation phase)
• Fitted wardrobe or storage installation
• Loft hatch fitting

Electrics (second fix):

• Socket and switch plates installed and connected
• Light fittings installed and connected
• Ceiling roses and pendants
• Kitchen appliance connections (oven, hob, hood)
• Bathroom pull-cord switches
• Consumer unit final connections and certification
• Part P notification or sign-off

Plumbing (second fix):

• Bathroom sanitaryware installation (bath, basin, WC, shower tray)
• Taps, shower fittings, and valves
• Radiator installation and commissioning
• Boiler commissioning
• Kitchen sink connection
• Dishwasher and washing machine connection
• Underfloor heating manifold connections and commissioning

Why First Fix Before Plastering?

The logic is straightforward: cables, pipes, and structural frames need to go inside the structure. Once the plaster is on, they are hidden and inaccessible. By completing all the routing and framing work before plastering, the contractor ensures:

• Nothing is missed (missed cable routes mean hacking into finished plaster later)
• Surfaces are clean and consistent for plastering
• The plasterer has a clear, unobstructed surface to work on

What Affects the First Fix / Second Fix Timeline?

The main factor is the drying time of the plasterwork. In a newly plastered extension or bathroom, plaster can take four to six weeks to fully dry in normal conditions — longer in cold or damp weather. Moving to second fix too early causes:

• Paint peeling as moisture migrates out
• Joint cracking in plasterboard
• Poor adhesion of tiles in bathrooms
• Swelling and sticking of internal doors

Good contractors build adequate drying time into their programme rather than rushing through it.

Materials and Who Supplies What

In RCB's standard scope, first fix materials (structural timber, cables, pipe) are supplied by the contractor or trade as part of the work package. Second fix materials — the visible elements (taps, sockets, light fittings, tiles, kitchen units, sanitaryware, door handles) — are client-supplied.

This is the standard arrangement across professional London contractors and gives homeowners full control over their finish specification. It is clearly stated in every RCB scope document to avoid any ambiguity.

RCB Design & Build

RCB programmes all first and second fix works in a structured sequence, with dedicated programme time for plastering drying, and clear communication to clients about which stage their project is at. Contact us to discuss your project scope.