7 steps to build a house extension

7 steps to build a house extension

Are you thinking of finally taking action to build your house extension? If you are a bit lost and don’t know where to start, keep reading our guide to discover how to build a house extension step by step.

This will be really useful for you. It may sound obvious to some, but many people ignore the steps they need to follow to build their dreamy house extension.

If you are still considering if building a house extension is the right thing to do with your home, have a look at the following article where I help you to make a decision.

N.1 - Find an Architect

planning application


I’m probably biased about this because I am an Architect, but I honestly think choosing an Architect for an extension is the first step you should take to build a house extension.  An Architect will tell you what is possible to be built on the specific case of your house. You probably have thousands of ideas, but not everything might be possible to be done in your house. Maybe you are not even considering other options that your Architect can present to you.

You will need to follow Council guidelines and Building Control regulations, so knowing what applies to your specific case is really important before you start chasing out builders. You can show around your house different builders, but they won’t be able to price accurately for the works you intend to do if they don’t have specific details about it. A fully tiled marble bathroom with a self-standing bath and a walk-in shower might not cost the same as a smaller bathroom with bathroom panels, right? It all depends. 

There are house extensions that can be built under the Permitted Development rights. There are others that would need a Planning Application. It all changes too if you live in a Conservation Area, if there is an Article 4 direction, or even what other applications were approved by the Council before in your area.


  1. Make a simple search in google looking for local Architects, talk with your neighbours and friends asking for recommendations.
  2. Filter them down by having a look at their website, their social profiles, and their projects. Do you have a good feeling about them? and even, how do they present themselves? Do you like what they do and how they do it? 
  3. Book a consultation with your favourtie ones to discuss your project. Choose at least three to compare.
  4. During the consultation, observe and make sure they get you. You will be working closely with your Architect and you don’t want to be disappointed with the result of your project. After all, it’s a big investment for you and for your family.
  5. Compare quotes taking into consideration what they offer, but also how you felt with them. 

N.2 - Get a Measured Survey to build your house extension

measured survey drawing


The next step to building an extension is to get a Measured Survey done. You can’t start working on design options if you don’t have the real dimensions of your house. Estate Agents’ plans are only an orientation and many times they are not even correct. Sometimes Architects offer this service too, that is why I left this as a second step. However, your Architect won’t be able to give you precise advice about your house if he or she doesn’t have accurate plans of the house as existing. 


Ask your Architect if they do measured surveys or if they can recommend someone to do it. Sometimes Architects have also a preferred format for the surveys to be made. It’s worth asking them what they need. Get three quotes in total from surveyors to make sure you have enough to compare. Extra fees will apply for this service independently of who makes it, so make sure you budget for it and compare quotes and quality levels.

N.3 - Permitted Development or Planning?

planning portal

Decide with your Architect what design option to build your house extension works best for you based on:

  • Design: your family needs and lifestyle.
  • Money: your budget.
  • Time: how fast you need to get your house extension done.


Depending on what is your priority, you might choose to go in one direction or the other. In some cases, you might not have the possibility to choose between the route of Permitted Developments or a Planning Application. However, in most cases, you can choose depending on what is your priority.


If your priority is to make sure the design works for you, just feel free to go crazy with your Architect and the design. He or she will let you know which route will apply to your house extension.


When deciding between Permitted Developments or a Planning Application, money shouldn’t be a problem. Although you don’t need to submit a Full Planning Application if you go with the PD route. Architects will generally recommend applying for a Certificate of Lawful Development to make sure the Council doesn’t have any objection against you to build your house extension. 

Either the Planning Application or the Certificate of Lawful Development has similar fees (although they have different timescales). The latest one is particularly useful to demonstrate compliance in case you want to sell in the future or in case you get any complaints from a present or future neighbour.


Time is of great importance when starting with the process of building your house extension. Usually, homeowners are not aware of how long is the process. 

If your house extension project needs a Planning Application, it will take at least 8 weeks to hear back from the planners. Sometimes more, sometimes less. there is usually a bit of discussion in case they have some comments and it could take them a long time to reply to your Architect’s e/mails and calls too. Having a proactive Architect is also key here to make sure the planners get what they ask for promptly. 

Going through the route of Permitted Developments is faster because you can submit the plans for a Certificate of Lawful Development at any point in time. It is optional and you can even do it retrospectively. Once you apply, it takes around 5 weeks to get the certificate and as I said, you don’t need to wait for the Council’s answer to continue with your project. 


Identify what is your priority from the ones listed above and discuss the details with your Architect. This will help you move ahead with the design and your Architect will submit the required applications when and if required.

N.4 - Party Wall agreements

party wall act 1996


The Party Wall Act. establishes how you need to communicate with your neighbour if you have a wall or a floor (in case of flats) that your share in which you intend to do works. You will need to communicate formally the purpose of the works and agree on how you will deal with any issue that might come up due to the nature of the works. It’s always best to have a friendly chat with your neighbour first. You will probably need a Party Wall surveyor too (you can share one with your neighbour too)


You can find a guide from the government here and you can discuss this with your Architect before proceeding. It’s usually required to inform your neighbours around 2 months in advance of the start of the works, but you can agree on an earlier date too. 

N.5 - Building Control Application to build your house extension

section with construction details


You might need or not to deal with the Planners, but you definitely need a Building Control Application when you are altering the envelope of your house opening into a house extension. This is important because building control will make sure the new space that you are creating has the characteristics to be safe, warm, and water-tight. 


Once you have signed off the design, your Architect will prepare the technical drawings to be submitted to Building Control. You can choose between dealing with the local department of your Council or you can appoint a private Building Control company. 

The second option is usually more expensive, but it tends to be faster and easier. It really depends on how fast they work in the Council and how many projects they have at the same time. In Councils where not many projects are being handled, they can also work perfectly fine and have the time required to pay attention to your project. You will need to account for the fees for the application as an extra.

N.6 - Tender out to Builders

schedule of works template


You might need or not to deal with the Planners, but you definitely need a Building Control Application when you are altering the envelope of your house opening into a house extension. This step to building an extension is important because building control will make sure the new space that you are creating has the characteristics to be safe, warm, and water-tight. 


Now is the time to go out and “hunt” a good Main Contractor or different trades to build your house extension. You will have in your power the technical drawings submitted to Building Control and therefore the builders can quote for a very specific job. Changes and unforeseen issues could always happen, but tender out to builders at this stage improves massively the chances of getting an accurate quote. 

If you want a great tool to help you break down the works so you can get a breakdown of the prices from each builder, I suggest you download my Free Schedule of Works template. You can fill out the template, attached the drawings, and send it out to the builders. They would be impressed with how organised you are and they will quote efficiently so you can compare them like-for-like and job-by-job. 

N.7 - Construction of your House Extension

Building contracts


Well, this doesn’t need much explanation, right? We did all of this work just to get to this point. The construction process will be stressful and difficult, but I assure you that is definitely worth the pain. After all, don’t you and your family deserve that great space to spend more time together? Focus on the end goal and keep organised. You can do this!


If you don’t have much time and experience, I suggest you get on board a Project Management. A professional with experience will be reviewing the contractor’s work and managing the contract. Make sure you have a contract in place even if your house extension is small and especially if you will manage the work by yourself. 

A contract will prevent misunderstandings about who buys what (if you buy materials, do you also buy the pipes that go through the walls? do you know which ones they are and where to get them?), how should be the communication between the parts (can they call you between working hours or do you prefer e/mails? should they go ahead if you don’t reply?), when are the invoices paid (on stages? until the works have been reviewed?), etc. 

Get organised and make sure there is good communication between all parts involved and you will succeed in the process. Some Architects include site visits during construction and can also be appointed as Project Management, but you can generally work with somebody else if you wanted to. Discuss this area from the beginning to make sure you understand properly the initial quote. 

modern house extension
Contemporary Rear Extension

If you have followed all the steps in this guide, I’m sure you will get everything under control to complete your house extension. That is all for now! I hope this guide has helped you to identify key areas that you need to consider when steps to build your house extension. Please, feel free to leave me any question you might have in the comments and I will you the best of lucks with your project!

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