Fulham, London - rear extension, loft conversion and entire house renovation including inserting swimming pool:   by Zebra Property Group

Home extension project planning guide

Johannes van Graan Johannes van Graan
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No two home extensions are alike, yet at the end of the day there are similar processes and timescales that can be applied to all of them, regardless of whether you’re extending your kitchen or adding a brand-new living room to your back garden. 

And like all things in life, mistakes are waiting around every corner – and it is up to you to find out how to avoid them. 

That is why we have drafted a schedule based on a single-storey kitchen extension which will give you an idea on project length, which you can use to scale up or down accordingly. 

Note that a two-storey extension will take between two to five weeks longer than a single-storey one. And if you plan on living in the house while the project is ongoing, things could also drag out a little longer…

Week 1 – 2: Setting out

The builders arrive on site and kick-off with the first steps for the new project, which will include digging footings and pouring foundations. 

Should you project-manage everything, the onus to arrange diggers will fall on you; otherwise, it’s the responsibility of your builders.

Week 3 – 4: Building up to damp-proof course

• Ensure that all 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

This will vary depending on whether the brickwork or blocks are built first; however, 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.

Week 6: Internal walls built

Now is the time to start 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

At this stage, a carpenter will be brought in to build the roof structure (who will also be in charge of fitting roof lights in most cases). 

Make sure that the roofers know when they are needed – you want your new structure to become watertight as soon as possible. Check that all roofing materials are in place, including the lead.

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.

Week 8: Installing windows and doors

Preferably your door linings and window frames will have been put in place while the external walls were built, which is why fitting the windows and doors can now be carried out.

Week 9: Electrics and plumbing

As soon as the extension is watertight, electrics and plumbing can get underway. 

The two trades often have to work closely together, which is why it’s important that both are always aware of what each other are doing to avoid delays.

Week 10: Break through from existing house

With the breaking through from the existing house, this is where things can get messy.

If you will still be living in the existing house for the duration of the project, this is where you need to seal yourself off from the mess as much as possible.

Week 11: Plastering

As a general rule, new plaster should be left for around a week before any decorating takes place. 

And seeing as plastering is a messy job, it is crucial that your plasterer has a clear route to a water source (preferably not your bathroom).

Week 12: Second fixes

Electricians, plumbers, floor fitters and kitchen installers need to work together so that each trade knows who is doing what and when – this is a stage where huge delays can occur quite easily.

Week 14: Snagging

Any leaks, electrical issues and sticking doors and windows need to be reported to the relevant trade as soon as possible after finishing.

Common mistakes to avoid

It certainly 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 errors: 

An unrealistic schedule – It is definitely true that a huge number of projects take longer than anticipated. Being aware of this from day 1 can help to avoid stress. 

Running out of money – Get fixed quotes where you can and try to have a contingency fund (around 10%). 

Picking bad tradesmen – Avoid your project turning into a series of arguments. 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 potential buyers could be put off by your house. 

Of course costs will also play a part; thus, have a look at: How can I plan and cost my single-storey extension?

What else helps an extension project run smoothly?
Whitton Drive:  Terrace house by GK Architects Ltd

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