How much will my extension cost?


When it comes to home-renovating the most common question is this one: What’s the cost of building an extension?

This question becomes especially crucial for those of us making the decision to move house while stamp duty is partially frozen, and to stretch to get that fixer-upper, the potential ‘forever’ home or the one with the bigger garden.

As a result of disruptive and ongoing world events, it feels like the best time to future-proof our living situations. We need not only a place our families can grow, but also space where we can work and, if need be, home-school. Now that a lot of different activities are happening under one roof, it’s no wonder we’re turning our attention to extending, refurbishing and improving our homes.


So, let’s get right to it. How much will it cost you to extend your home? And how do you work out the REAL cost?

There are FOUR different ways you can tackle these questions:

  1. Find a square metre cost
  2. Use a cost calculator
  3. Get an early cost estimate from a builder
  4. Create a budget breakdown

In this article, we’ll go through each strategy, highlighting its pros and cons, so that you can plan for the future with confidence!

*SPOILER ALERT* Making a budget based on real project percentages is the best way! Skip to Strategy 4 if you want to save time.


As a rough estimate of construction costs, you can expect to pay:

  • Basic quality: £1,320 to £1,620/m²
  • Good quality: £1,700 to £2,000/m²
  • Excellent quality: £1,800 to £2,500/m² [1]
  • High-end bespoke design: £2,500-3,000/m² or more.[2]

And as a rule of thumb, an excellent finish will cost you 40% more than a standard finish.[3]

BUT (and it’s a big but, I’m afraid) there are a lot of caveats with these prices!

1. Location - where you are based in the UK will change the square metre rate. For example, if you are in London, be prepared to pay more.

2. Scope - what you want to build will obviously make a difference to the square metre rate. If you’re going for a two-storey extension with marble worktops and a polished concrete floor … that’s going to cost more than a “box on the back”.

As an aside, some experts might say that you get economies of scale (cost advantages) if you do a larger extension. In fact, some may say “twice the height, half the price”, but this is sadly a little bit of a red herring. Work out your overall floor space and ensure your budget matches.[4]

3. Team – the professionals that you get on board to help you will potentially impact the overall costs of the project.

4. Involvement – how involved you want to be in the process will affect the costs of the project too. For example, if you are a pro DIYer and choose to source and install the kitchen yourself as opposed to outsourcing this to a kitchen company, that decision will impact the cost.


Cost calculators are tools you can find online to help set your target budget. They can be beneficial, especially to work out a rough construction cost. However, you need to be really careful when using these tools. They are only as reliable as the information you put into them!

Check out our top three favourite cost calculators below:


This simple cost tool by Urbanist Architecture is good for a quick ballpark calculation. It’s based on projects in either central or outer London.

It gives you an overall estimate per square metre for either a refurbishment or new-build extension. 

The calculation includes all construction costs, clients’ selected items and basic consultants’ fees and allows you to select the level of finish for your project.

This costing tool also gives a quick ballpark calculation. It uses your address to tailor prices to your region, and takes into account the number of storeys and quality of your build.

It gives you an overall estimate per square metre that includes all construction costs, clients’ selected items and VAT. It does not include consultants’ fees.

We advise to use this calculator on its highest settings to give a more representative price of a design-led project in London.


London-based 2PM Architects have developed another helpful guide.

It includes costs per square metre for different levels of specification (basic, medium and high) and is based on 2020 prices.

It also has an online cost calculator that, based on your answers, immediately gives you an idea of prices and simultaneously displays your project per square metre rate.



This can be a really great way to get an idea of costs in the early stages of thinking and strategising. But remember that it’s exactly that – an idea of costs. One of the drawbacks of getting an early price from a builder is that that becomes the figure stuck in your mind – even if you decide to do more than you initially intended!

But if you want to go down that route, here are a few tips to bear in mind:

1. Ask what the price includes

Often the builder will tell you a price that is only the construction cost. But remember you also have VAT, contingency and professional fees to consider! Plus, it’s unlikely they’ll include client supply items like your kitchen or sanitary ware.

2. Ask what work the builder has on his plate

Prices will change dramatically depending on the market. If your builder needs the work, then he’ll go lower to secure the project. If he’s flat out and doesn’t need the work, then it will be higher!

3. Do your research

Make sure you’ve done your homework on the contractor you get round for a quote. Ideally, it’d be a builder who is working on projects similar to what you have in mind. So, if you’re looking to achieve a ‘Grand Designs’ forever home, then you need to find a builder who suits those plans


It may come as no surprise that we advocate making a detailed budget. It’s not the easiest, quickest or sexiest-sounding solution! But if you want a more accurate breakdown of the costs of your extension, making a detailed budget is the way forward.

It’s safe to say that most people renovating their homes won’t be able to afford everything they want to include. Renovating your home is a balancing act between what you wish to create and what you can afford.

When the budget is tight, this balancing act becomes even more critical.

So before you do anything else, it’s essential to get to grips with the different aspects of a construction budget and how these elements work together. This will help you make some fundamental decisions before the project has begun.

We have created a FREE Renovation Budget Breakdown Guide that unpacks all the figures of a project – from building costs and professional fees to those extra expenses that no-one tells you about.

This will put your construction budget in the context of the whole project’s costs and make sure you’re considering all those added extras from the outset.

If you want more hands-on budgeting help, you can sign up for a HomeNotes membership. You’ll receive instant access to a library of masterclasses that covers everything you need to know about renovating your home!




[2] London Architects Group (




Our HomeNotes Journal is here to bring you clear and up-to-date information for homeowners renovating their homes. For more in-depth help we provide online courses to help you get started the right way!

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