29 Jun 2023

The Dos and Don’ts of Planning a Home Extension

Here, RIBA-chartered Architect George Omalianakis from architect firm GOAStudio offers professional advice, tips and key insights to guide you through the process of adding an extension to your home.


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Design matters:

What do I want, and how can I achieve it?
Remember, B comes before D but not before A. You need a brief before starting with the design process because you need to know in advance what you hope to achieve and your budget for your home project. However, you will only be able to finalise your brief after you have appointed an architect to advise on the overall feasibility of your project, which will then inform your brief and help you make a final decision on how best to proceed.

Do design from the inside out. Start by thinking of the internal spaces and how they work for you, including the existing spaces next to the new extension. Adding an extension means that everything else changes too, and you will need to tweak things to ensure that everything, new and old, works together in harmony and to your needs.

Do think long term with the choice of materials and the associated plumbing, wiring, heating and insulation you can add to your home. You will probably have one opportunity to upgrade the fabric and systems of your home. When planning your extension, consider what other upgrades you would like to do and how your budget could stretch to cover these additional works.

Approval matters:

What approval will I need for my home project?
For your home extension project, you are likely to need one or all of the following approvals:

1. Planning: Full planning, a householder application, a permitted development application or listed building consent depending on the type of your property (house or flat), the type of work you would like to carry out and where you are in the country. This is an approval you need to obtain from your local authority and remember that there is a number of different applications under the umbrella name of ‘planning’. Ask your architect and local planning authority to advise you on which application you need to submit for your project.

2. Building Regulations: This approval is in regard to a number of technical and structural standards that need to be met to ensure safety and compliance. This includes fire safety, thermal insulation, sound insulation and structural considerations. You do not need to have detailed knowledge of any of these matters. Still, you do need to make the right appointments (typically an architect, technician or technologist and a structural engineer) for knowledgeable professionals to prepare this information on your behalf.

3. Party walls: If you do works near a shared wall, floor or ceiling, then the Party Walls Act might become relevant to your project. This means that you will need to engage with the neighbours under the act and satisfy them that the works will not affect their property. Do contact a party wall surveyor who will give you all the advice you need.

4. Freeholder approval: If you own the leasehold of the property or a share of the freehold, then you will need this approval from anyone with a freehold interest in your property. Do check the relevant terms in your lease or share of freeholder agreement, and ask a solicitor to advise you on the process you must follow. This process is specific to you and the particulars of your agreement.

5. Water authority: If you do an extension near or over any public drains in your garden, then you will need to obtain the relevant approval from your local water authority. Do appoint a CCTV drain survey company to survey and map the private and public drains around your home. Do send this information to your local water authority, who will advise you on whether you need to apply or not, and how to go about it.

Do apply and do obtain in advance all the approvals you need for your project. Do not take chances because this will cost you later in money and time.

Money matters:

How much will it cost, and what can I do with my budget?
You do need a shopping list of project requirements before you can have a realistic and reliable estimate of costs for your home project. No builder, architect or quantity surveyor will be able to tell you how much your extension will cost before they have a list of design, technical and structural information to price.

Do start with a budget, though, and decide how much you are comfortable spending for your project. This is a much better starting point when you start planning your home project. Your budget will determine what you can and can’t do. Ultimately, you will have to cut your cloth accordingly during the design development stage.

Be prepared for the possibility that your project requirements and budget are incompatible and that something has to give in this case. You can set priorities for the budget and then stick to these priorities for your project. Alternatively, you can re-think the budget, decide that some things are too important to leave out and magic something to happen to increase the budget accordingly, if possible.

Do prepare some initial design proposals and do get some rough cost estimates from a builder or quantity surveyor. If the estimates are roughly in line with your budget, then you can further develop the proposals and make the planning submission to the council.

Do the same once you have prepared the required technical and structural information for the Building Regulations submission. Is the new estimate still in line with your budget? If so, you can make your final decisions on finishes and electrical before sending the information to the contractors to tender for detailed quotes.

Time matters:

How long will it all take?
Be cautious about how long things might take. The chances are that everything will take a bit longer than expected. Often this is due to the fault of no one in particular. Sometimes, this is due to ‘unknown unknowns’ that you only discover later in the day.

You should always have a contingency plan and be ready when things overrun. You often read that you should have a contingency regarding costs. Similarly, the trick is that at the start of your project you also have a contingency plan when it comes to time, which will take a lot of stress out of the whole process.

Generally speaking, it takes around four to six months to go through the design development phase and to obtain all necessary approvals before your builder can start on site.

Construction matters:

What are the key things to know during the construction stage?
Do have a written and robust contract with your builder. In the event of a dispute, you will not be able to rely on the memory of what was said, who said it and when it was said. A lack of a robust contract is an actual recipe for disaster and problems down the line. A fair contract will keep things straightforward during the construction stage. A written contract, and the clarity it provides to all parties, will give you peace of mind.

Do agree on a payment schedule with your builder in advance. Typically, it should include four to five stage payments. The first payment should be no more than around 20% and include a retention fee (typically around 5%) that is not paid until every single piece of work has been completed, including any snagging items.

Further information....

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