Week-by-week extension planner

If you’re planning to extend your home, use this step-by-step guide and handy checklist to ensure your extension project runs smoothly

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This glass box extension to a terraced home has created a light filled kitchen-diner

Week 1 – Preparation

  • Make sure there is clear access to the site
  • Have bricks, blocks and other building materials delivered and stacked close to where they will be needed
  • If you are project managing, hire a mixer and digger, otherwise your builder or contractor will do this for you
  • Make sure the site is safe, particularly if you have small children

Week 2 – Groundworks

  • Your builders will arrive on site and dig the foundations
  • Building Control need to visit the site to approve the foundations
  • If required, reinforcements will be laid within the foundations
  • Pipework, drainage and services will then be laid within the foundations
  • Concrete footing will be poured in and levelled
  • Building Control will need to visit again to approve the footing

Weeks 3-4 – Superstructure

  • Bricklayers will build up the damp-proofing course
  • Drains will be installed and trenches dug for the pipework
  • Concrete lintels will be inserted into the brickwork, if required
  • Sand will then be laid before damp-proofing membrane is put down
  • Insulation will be fitted
  • The concrete slab for flooring will be poured
  • Building Control will need to carry out another inspection

Week 5 – External walls

  • Check that lintels, door and window frames and wall ties have arrived
  • Your builder will create the blockwork and brickwork
  • Cavity wall insulation will be fitted
  • Wall ties will be inserted to fix the new walls to existing ones
  • Lintels for windows and doors will be fitted
  • Door and window frames will be inserted as the walls are built

Week 6 – Internal walls

  • Internal walls will be built
  • Make sure you have ordered the windows, doors and roof tiles as these sometimes have lead times of up to five weeks
  • Check that your carpenter is ready for the following week and all materials are ready, including lead

Week 7 – Roof structure

  • Your carpenter will start building the roof structure. Sometimes, prefabricated roof trusses may be used
  • If you’re having rooflights or dormers, theses will be created and fitted

Week 8 – Roof covering

  • Roofing membrane will be laid over the new rafters
  • Roof battens will be cut and fitted over the membrane
  • Tiles/slates will be laid
  • Ridge tiles and valley tiles will be laid, along with finishing details like flashings
  • Fascias, sofits and verges will be primed, stained and painted
  • The floor screed will be laid

Weeks 9-10 – Windows and doors

  • External rendering will be completed, if required
  • Windows and doors will be fitted into the frames that were put in place when the walls were build
  • Guttering and drainpipes will be fitted
  • First-fix carpentry, plumbing and electrics will be completed
  • Studwork walls will built with door linings fitted and pipes boxed in

Week 10 – Breaking through

  • Now is the time to seal off the extension from the rest of the house as things are about to get messy
  • Steels will be put in place, along with padstones that have been approved by Building Control
  • Joins will be made good

Week 11 – Plastering

  • Walls will be boarded with insulation placed between battens on the existing internal walls
  • Plastering will take place and must then be left around a week before decorating can be carried out

Week 12 – Second-fix

  • Second-fix electrics will be carried out, which means sockets will be made live, switches will be put in place and lights fitted
  • Second-fix plumbing means taps will be installed and connections completed
  • Flooring will be laid
  • Kitchen units will be installed, if applicable

Week 13 – Snagging

  • Report any issues such as leaks, electrical problems, heating system issues, or sticking doors and windows to the relevant trades to get them fixed as soon as possible

Extension checklist

If you’re managing the project yourself, use this handy checklist to ensure you don’t forget anything:

  • Obtain planning permission, if required, prior to work starting
  • Submit and application to your local Building Control department
  • Organise the hire of plant, toilets etc
  • Get quotes from trades and let them know if you will be completing any tasks yourself
  • If you’re planning to live on site, arrange the schedule to minimise the impact on daily life
  • Agree timescales and schedules with trades to avoid delays
  • Arrange insurance
  • Let your neighbours know that work is about to start
  • Arrange access for delivery lorries and decide where skips will be placed
  • Set up accounts with your local builders’ merchants
  • Check delivery times for materials and order when needed
  • Notify Building Control that work is starting
  • Make space to store materials safely on site
  • Ensure water will be available for cement mixing and plastering
  • Make second-fix decisions. such as the positions of lights and sockets, as early as possible
  • Allow time for plaster to dry before decorating