How to Build a House Extension Step-by-Step

Whether it’s to accommodate your growing family or add value to your home for reselling, the addition of an extension to your house can offer many benefits to you as a homeowner. Home extensions can help change how you live without having to spend more to move to a bigger space. However, it can be difficult to know how to build a house extension step by step without the proper information.

Adding a house extension entails careful planning and having all the requirements to ensure everything is in order and you don’t run into trouble later on. With this step by step guide, we aim to help you understand how to build an extension on a house and learn all the necessary requirements when doing so.

Building an Extension – Where to Start

planning an extension

  • Do your own research

The first of the steps to building an extension, you will first need to do your own research about it. You can start off by asking a local real estate agent and asking them what kind of house extensions can add more value to your home. This is especially necessary if you’re planning on reselling your house later on.
If you know someone who had an extension done, you can also ask them for their advice and recommendations. Another way is to survey your neighbourhood and check out what kind of extensions are typically done there. You can also check out this guide to different types of house extensions for more information.

compute the cost

  • Compute the costs and establish a budget

When undertaking any home improvement job, the cost of it is always a factor. This is why when planning a house extension, you will also need to consider the cost of it. The cost of your extension will depend on a variety of factors such as the complexity of the design, the materials used, and the overall quality of the design.
Generally, around £1500 to £2000 per square metre is to be expected for this type of home improvement. However, it can go up to as high as £3000 per square metre as well. However, keep in mind that house extensions can increase your home’s value by up to 25%.
Once you’ve computed the costs, establish a rough estimated budget for it. Consider alternative financing options such as a personal loan or mortgaging your house. 

  House Extension Process – How to Plan An Extension to Your House

Once you have the necessary information, you can now proceed with the rest of the process starting with:


  • Checking out if you need building permits.

Since not all house extensions need permission but some still do, it would be best if you check with your local authority if the extensions you plan on building requires you to apply for permits. You can also check out the criteria here and see if you’re required to apply for permits or not.

There are house extension builders as well that offer bundled services as we do. Bundled services can make the process easier because the builders you hire can manage everything for you – from the extension plans to the permits needed as well as the construction of the whole thing.

  • Ensure you comply with building regulation

Whether or not you need permits from your local authority, you will need to comply with the local building regulations to ensure the proper building of your extension. Your plans should cover issues like drainage, insulation, and fire safety.

There are two ways to go about it: a) submit a Full Plan Submission to your local authority and pay for an inspector to check the construction periodically, or b) submit a Building Notice to the council that you intend to comply with all regulations upon starting the job in 48 hours.

If you want the most convenient and least risky way, then go with the first option. Again, some builders, like us, offer bundled services that handle the entire process of building house extensions – including the legalities of the process.

drawing design an extension
house extension over structure

  • Find your architect and builder.

Whether you’re going for something simple or more uniquely you, you will need an architect to design the plans. However, when hiring an architect, make sure that they have all the necessary qualifications for the job and they also have professional indemnity insurance.

After you have hired an architect, you need to find a reliable builder to undertake the construction. While you can choose to oversee the project yourself, it can be difficult, especially if you have a full-time job and a busy schedule.

To assist you with this, choose a builder you can trust and who can deliver your desired results in a timely manner and stick within the established budget. You can start off with recommendations from friends, family, and colleagues.

Don’t forget to check the builder’s certification and insurances to ensure that they are qualified to take on the job. You can also check out their past work and see if they’re the right fit for your project.

While you can hire your architect and builder separately, there are house extension builders like us who offer packages like our Design and Build wherein our highly-skilled architect and team of expert builders can work closely with you and guide you throughout the entire process.

  • Find alternative accommodations in the meantime.

Adding a house extension entails huge and disruptive construction work so it’s best if you stay with friends or family in the neighbourhood during the height of it. If you don’t have any in the neighbourhood, book accommodation but stay in the area. This way, the builder can contact you and show you if there are any adjustments to be made.


Adding an extension to your home can greatly increase its value and it can improve how you live in your home. The added space can accommodate your growing family’s needs and give you more space to move in and breathe. However, undertaking this type of home improvement project requires you to take the proper steps which is why we have created this step by step guide. With this, you can follow the proper procedure when it comes to building an extension for your home. 


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office@marriottconstruction.co.uk

We operate across all of North London and North West London including Hampstead, Mill Hill, Finchley, Edgware and Barnet. We also cover parts of Hertfordshire including Bushey, Elstree, Potters Bar, and St. Albans.