Typical timeline

A common question during the planning stage is “how long will my extension take to build?”. As an example, we’ll use the popular 25-30m2 single story flat-roof extension to show typical durations. You should expect a timeline of 16 to 18 weeks for completion to paint finish. Allow an extra 4 weeks if you are building a second storey. In addition, allow an extra 4 weeks if the second storey is to be fitted out to paint finish.

The key stages are broken down below.

Stage 1: Super structure

This first stage results in a weather-proof shell, ready for integration with your existing building.

Duration Task
2 to 3 weeks FOOTINGS AND DRAINAGE
Excavate ground and remove waste
Dig footings and pour concrete
Pour Insulated floor slab
Build blockwork out of ground up to damp course (DPC)
Install soakaway if required for additional volume of rainwater
Make foul drain connections with inspection chambers at change of direction
2 weeks BRICKWORK
Build insulated block and brick cavity walls up to roof plate
2 weeks ROOFING
Build timber roof rafters and joists
Roof tiling or single-ply flat roof covering
Fit fascia and soffits
1 week GLAZING
Install double glazed windows and doors

Stage 2: Steelwork and first fix

After that comes the integration of both structures and the introduction of services

Duration Task
1 week KNOCKTHROUGH
Temporary support works installed
Walls removed to integrate into the main house
1 week STRUCTURAL STEEL AND CARPENTRY
Steel beams to support knocked through walls
1 week 1ST FIX ELECTRICS AND PLUMBING
At this stage cables and pipework will be run into walls and ceilings, before they are boarded up and plastered

Stage 3: Second fix to plaster finish

Next, the interior spaces are plastered and the services are terminated

Duration Task
2 weeks DRYLING AND PLASTERING
Fix plasterboard to new walls and ceilings
Plaster skim ready for paint finish
2 weeks 2ND FIX ELECTRICS AND PLUMBING
Fit electrical light fittings and power sockets
Fit plumbed radiators and connect to boiler
2 weeks 2ND FIX CARPENTRY
Hang internal doors and fit skirting/architrave
Prepare floor for finished covering e.g., LVT, timber planks

Stage 4: Fitting and finishing

Finally, the units and appliances are installed and the space is decorated.

Duration Task
2 weeks RE-MODELLING
If more extensive remodelling in your plan, build stud walls, run services, make good floors and ceilings
2 weeks INSTALL KITCHEN, BATHROOM, UTILITY, WC
Install fitted furniture and fittings
2 weeks FLOORING AND DECORATING
Lay new finished flooring
Prepare and paint new walls, ceilings and woodwork