When adding an extension, the order of works will vary from project to project, but this planner will give you a good idea of what is involved.

There are also common mistakes to be avoided, so after we have outlined the order of works, you can read on to find out how to ensure your project is successful.

The schedule below is based on a single storey kitchen extension. It will give you a rough idea on project length and allow you to scale up, or down, accordingly. A two storey extension takes between three to five weeks longer than a single storey design. And, if you plan to live in the house while the work is carried out, things tend to take longer.

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The Project Schedule

Week 1–2

Setting out, footings dug, and foundations poured

  • Builders will arrive on site.
  • If you are project managing, you may need to arrange digger hire, but your builders should do this for you. 
Setting out, footings dug, and foundations poured

Week 3–4

Building up to damp-proof course (DPC), drains installed, DPC installed and concrete oversite/floor structure put in place

  • Ensure materials are ordered and delivered ready for the concrete floors to be poured, including insulation and any damp-proof membrane.
  • In some cases, a drain run may require that a concrete lintel will need to be inserted into the brickwork at this stage.
  • Sand will then be put down before the damp-proof membrane, insulation and the concrete slab is poured.

Week 5

Building of external walls

  • It varies whether the brickwork or blocks are built first, but in both cases cavity wall insulation will be fitted at this point.
  • Wall ties are used to fix the new walls to those in the existing house.
Building of external walls

Week 6

Internal walls built

  • It’s time to begin thinking about arranging delivery of items further down the line which can be on lead times of three to five weeks (e.g. windows, roof tiles).

Week 6–7

Roof structure built

  • A carpenter will be brought in at this stage to build the roof structure. In the case of rooflights being fitted, they will also be in charge of this in most cases.
  • Ensure the roofers know when they are needed — you want to get watertight as soon as possible.
  • Check all materials are in place for the roofing, including lead.
Roof structure built

Week 7

Roof Coverings

  • Roofing membrane will be laid out over the rafters before roof battens are cut and fitted over — how they are spaced will depend on your roof covering of choice.
Roof Coverings

Week 8

Install windows and doors

  • It is likely (and certainly preferable) that door linings and window frames will have been put in place while the external walls were built, so fitting windows and doors can now be carried out.
Install windows and doors

Week 9

First fix electrics and plumbing; carpentry

  • Once the extension is watertight, electrics and plumbing can get underway. The two trades often have to work closely together, so ensure each knows what the other is doing to avoid delays.
First fix electrics and plumbing; carpentry

Week 10

Break through from existing house, steels put in place, joins made good

  • With the breaking through from the existing house, this is where things can get messy and, if you are still going to be living in the house, you should start sealing yourself off from the mess as much as possible.
Break through from existing house, steels put in place, joins made good

Week 11

Plastering, followed by a period of drying out

  • As a general rule, new plaster should be left for around a week before any decorating takes place.
  • Be aware that plastering is a messy old job and that your plasterer will need a clear route to a water source (preferably not your bathroom…).
Plastering, followed by a period of drying out

Week 12

Second fix, electrics, plumbing, flooring fitted, and kitchen units installed

  • You really need to ensure all these processes can work together and that each trade knows who is doing what and when, as this is a stage when delays can occur.
Second fix, electrics, plumbing, flooring fitted, and kitchen units installed

Week 14


  • Any leaks, problems with electrics and sticking doors and windows need to be reported to the relevant trade as soon as possible after finishing.

Ensuring Project Success

Avoiding Common Mistakes

It helps to be aware of the most common mistakes made by extenders if you are to avoid making them yourself. Here are some of the top extension pitfalls.

Being Unrealistic with Your Schedule
Ok, so you might be the exception to the rule, but a huge number of projects take longer than anticipated. Being aware of this from the very beginning can help to avoid stress.

Long Planning Delays
Having a chat with the planners before submitting plans can help avoid delays and disappointment. Find out what is likely to be acceptable and you will be able to get an idea of whether or not you are going to run into problems.

Running out of Money
Get fixed quotes where you can and try to have a contingency fund (around 10 percent). There is nothing like a half-finished project, ground to a halt by an empty bank account, to get you down.

Being Indecisive
We all change our minds and it can sometimes be the case that once a project is underway, part of the design no longer seems the good idea it initially did. However, sticking to the original plan as much as possible will avoid delays and overspending.

Choosing Bad Tradesmen
Don’t let your project turn into a series of arguments and let-downs. Choose tradesmen based on their previous work and trusted recommendations — not on the cheapest quote.

Forgetting Bathrooms
If you are adding bedrooms, you may well need to add a bathroom, or at least a shower room. If you are having more than three bedrooms, you will need more than one bathroom — otherwise future buyers could be put off the house.

Getting Windows Wrong
A surefire way to ruin any project is to get the window design wrong. If you are trying to build an extension that matches the original building, then match the windows exactly. Don’t think that plastic look-a-likes will suffice next to timber — they won’t.
Likewise, consider the impact that the new extension will have on the existing windows and light. Making existing windows smaller to accommodate a new roof, for example, can mean things start to look out of proportion.

Keeping Things Running Smoothly
There is plenty you can do to ensure that each stage of your project gets completed with minimal fuss, meaning the next stage can commence without a delay, and the stress on you and your family is reduced. Things that may seem low down in terms of priorities on your to-do list can actually make all the difference. Take the provision for toilets, for example. You might think you are ok with tradesmen using yours, but you may change your mind after weeks of having builders traipsing in and out of your already disrupted home with muddy boots and dusty clothes — making a tricky situation worse. Some contractors will provide their own portable chemical toilets, which cost around £45 per week to hire.

You also need to remember the following:

  • Regularly hiring skips will also help keep building-related rubble under control, rather than letting it pile up into huge unmanageable and unsightly mounds. 
  • You should also be aware that tradesmen will require access to water and electrics.
  • Make sure you are in regular communication with your builders about what is happening and,what will be happening next — especially if you are acting as project manager. Knowing that the electrician or plumber will be required before work can continue on an aspect of the build, or that materials are going to be needed next week, is vital if things are to keep ticking along at a good pace. 

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