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The process to building an extension

From planning your budget upfront to planning applications and finding the right contractors, here's everything you need to know about the process to building an extension.

6 min read

© Matt Gamble

Building an extension isn’t a walk in the park but the rewards that come with renovating your home can be transformative. Knowing where to start and what to expect are some of the greatest challenges you’ll face in your extension journey. But we’re here to help with each step of the way! We’ll discuss everything from planning your budget to choosing the right contractors for the project and how you might include energy efficiency for your extension.

Planning your budget upfront before you start

When it comes to building an extension, budgeting for the project is one of the most important factors and can have a huge impact on how smoothly it goes. Whether it’s your first time taking on a home renovation or building an extension, we’re here to help guide you from start to finish.

What factors should you take into account in the overall cost of the extension?

There are a number of costs to consider before you undergo a house extension. We take a look at some of the most significant factors and how you might begin to estimate them.

Cost of your desired house extension

While this in itself depends on multiple factors, it can be helpful to get a ballpark estimate to have a sense of how you might finance building an extension can be incredibly useful. You can use our Quick Quote Calculator to detail what extension type you’re considering, plus a few more details to get an idea of how much it could set you back. Alternatively, you can book a free advice call to speak about your proposed extension ideas in more detail.

Location of your property

The location of your property will have an impact on how much your overall extension costs to build. As a general rule of thumb, building an extension in a capital city is more expensive than in less populated areas. However, in both areas you may struggle to find contractors to fulfil your project either due to competition or lack of professionals in your area.

Admin fees

If you’re not able to build an extension under Permitted Development at your property, you may have to seek Planning Permission. This process in itself comes with a set of fees for applications and may require various other services along the way – for example, detailed architectural drawings.


Building a house extension takes a village. Depending on the nature of your proposed build, you’ll need a number of contractors to bring it to life. This can include builders, electricians, plumbers and surveyors. Doing your research, calculating how much each is likely to cost and setting a portion aside will benefit you later on.

Soil, drainage and foundations

To build a solid extension that’s built to last, it’s really important to get the foundations right. That means getting elements including your soil, drainage and existing foundations assessed and resolved, if necessary, before your build can begin. Each of these assessments and improvements involve a trained professional and should be considered when you’re outlining your initial budget.

Contingency planning

The reality of home renovations and extension projects is that they often end up costing more or taking longer to complete than originally anticipated. We recommend setting aside a contingency amount in case any unexpected costs need to be accounted for.

Planning your budget to building an extension© Matt Gamble

How can you finance your extension?

There are a number of ways to finance your house extension project and each option comes with its own set of pros and cons. We’re going to look into a few of these options and weigh up the pros and cons for each.


If you’re in a position to save money, this could be a great way to help fund part of your home extension. That being said, extension projects rarely begin at less than £20,000 and the total amount could be significantly more. Alongside personal savings, many people choose to pay for instalments with a credit card.


If you’re in a position to pay back any credit card payments promptly, this method could offer you a 0% interest route.


Saving from scratch takes time and you might find that by the time you’ve squirrelled enough away, inflation has impacted the cost of the labour or materials that are needed for your renovation project and you’re still short. You could also find that by the time you’ve managed to save enough money, the size of your extension is no longer suitable for your household’s needs.

Personal loan

This method is more suitable for small projects as larger ones can have dizzying costs and the repayment schedules that come with borrowing a significant amount can be extremely difficult to manage. If you have a good credit rating and are able to find a good rate, it could be a suitable option for you.


If you can find a short-term plan that you’re able to pay back, you can fund your extension and avoid large set up fees.


On larger loan amounts, you could run into high repayment amounts that can leave you in the red for longer. Additionally, if you don’t have a great credit rating or if the market you’re looking to loan in is struggling, it could be challenging to find a reasonable rate.


Remortgaging relies on the existing equity of your home to obtain the funds you need. This is a great option for larger or more complex projects and could secure you a low interest rate for your repayments.


For large home renovation or extension projects, this option can provide homeowners with the most competitive long-term interest rates.


It’s possible that you might end up with a repayment rate that’s significantly less favourable than your existing mortgage.

Preparing for building an extension© Matt Gamble

Where to start

Preparing for planning permission or permitted development

First things first, identify whether you need to obtain planning permission or you can carry out your proposed project under permitted development rights. You can find this out by booking a free advice call with our experienced Planning team or contacting your Local Planning Authority (LPA).

Building regulations

Any lawful build in the UK must comply with building regulations. To make sure yours fall within these regulations, get technical drawings or your proposed project so your contractor has clear guidelines for how to observe all legal requirements.

Getting neighbour approval

Some renovations or house extensions require you to get permission from your neighbour if they fall under certain restrictions – specifically if you share a party wall with them. If you’re planning to build on or below this boundary, you must let them know. In this circumstance, you’ll need to get a party wall agreement that allows you to proceed with the work.

To secure this, you must serve party wall notice and offer your neighbours the chance to reject or approve your plans. The most effective way of getting this approved is to let them know ahead of time and maintain a cordial relationship with your neighbours. You’d be surprised how far taking a box of chocolates around from time to time can get you…

Finding the right architect

Why you should get one

Whether you’re planning a relatively small or a large-scale renovation or extension project, an architect can significantly enhance how smooth the process goes. It’s not a legal requirement to hire an architect but their experience and knowledge of structure and materials can set your project in the best stead.

How to find the right contractors/builders

It can be tricky to find the right contractors for the particulars of your home renovation or extension project. That’s where our Connect service comes in handy – this service introduces you to a pool of vetted contractors in your local area that might be suitable for the work you need done. Learn more about our Connect service and explore some of the professionals in your area here.

Including energy efficiency in your project

As we recently learned from the results of our 2023 Happy Homes survey, one of the most important aspects of happiness at home is how much it costs to run, as well as the temperature of your home in winter. These two factors, alongside the continued climate crisis, are encouraging heightened awareness of energy efficiency in all corners of our lives and our homes are no different.

If you’re looking to renovate or extend your home in the near future, it’s also a fantastic opportunity to retrofit. This is something we’re going to hear a lot more about as time goes on. Retrofitting is the process of better equipping your home for energy efficiency – from better insulation to improved ventilation. If you’re looking to have work done to your home anyway, it could be a great opportunity to improve its energy efficiency at the same time.

Looking to improve your home?

Which part of your property would you like to extend?

Ground Floor
First Floor


What order do you build an extension?

One of the first steps towards building an extension is to get your existing property surveyed for soil, drainage and its foundations. The next step is to have any outstanding issues that have been identified addressed. Next, we recommend getting detailed architectural drawings of your existing property done and drawings of your proposed build. These will prove helpful for later down the line, particularly if you’re hoping to secure planning permission.

What planning do I need for an extension?

Depending on the proposed plans for your renovation or extension project, as well as your existing home and the area you live in, you’ll either be able to build under permitted development rights or seek planning permission.

What do I need to do before building an extension?

There are a number of steps we recommend you follow before building your house extension. These include securing permitted development rights or planning permission, getting a building regulations package, having technical architectural drawings of your existing property and proposed build done and obtaining a party wall agreement.

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