A house extension can be the best investment you ever make. An extension can add far more value to your house than you spend on it. But the extension cost needs to be right. We want our clients to be happy with what they spend, and get max value for money. These are 3 things to keep in mind:
- It helps to get a handle on costs early, and set a budget for the work. Let’s look at some average ball park costs for similar work. Use the Build Cost Calculator below for a first estimate.
- It’s good to update the cost estimate with design and spec choices. You can decide if the budget should change to suit the estimate, or if the design needs to change to hit the budget.
- The build cost will depend on choosing the right builder. Getting fixed costs from a good builder is worth its weight in gold. The key is to tender with detailed drawings and spec. The truth is builders prefer this too.
Average Extension Build Costs 2018
|Average Extension Build Costs In £ per m²|
|(Costs for 2018 in London)|
|Basic Spec||Medium Spec||High Spec|
|Single Storey Extensions||£1,250||£1,850||£2,500|
|Double Storey Extensions||£1,050||£1,650||£2,250|
So what is the total cost of a house extension? The short answer is that the cost will depend on the type, size and spec of the extension. So it can be useful to look at some average costs per m² to set a budget. Costs need to be up to date and for the right part of the country.
London build costs have gone up a bit in 2018. The build cost for a simple extension can be £1000 – £1500 per m². For glass extensions or high spec design £2000 – £2500 per m² is more likely. For basements costs can be £3000 + per m².
Single Storey Extensions
The cost of a single storey extension can vary from £1,250 per square metre for a simple build and spec, to £2,500 per square metre for a high spec build. Single storey rear extensions or side return extensions will often include the cost of a new kitchen. Check out the cost calculator below to factor this in.
Two Storey Extensions
Costs for two storey extensions can be a bit less per square metre than single storey. This is because they share the below ground work. The cost of a two storey extension can vary from £1,000 per square metre, to £2,200 per square metre for high spec work.
Basement Extensions and Refurbishments
Costs for basement extensions are more than for single storey or two storey extensions because of the ground work. Most basement work will be at least £3,000 per square metre. The costs of interior refurbishment can vary from £500 per square metre to £2,000 per square metre for a full refurb.
How To Set An Extension Budget Cost
A good architect will be able to help to show anything that could affect the build cost. They can also help to make clear what needs to be included in the budget. For example, if you add a kitchen or bathroom to the extension this can change the costs a bit. It’s normal for build costs to be given ex VAT (as VAT can vary) so you will need to factor that in too. You also need to add on for Planning fees, Building Control and Party Wall awards.
Extension Budget Cost Breakdown
- Build Cost
- Planning and Building Control
- Party Walls
- Architect, Engineer, QS
The Build Cost (or Contract Value) will make up around 55% of the budget for a house extension. Add a contingency and VAT and this will take it up to about 80%. You need to set aside about 5% for Planning, Building Control and Party Walls. The last 15% is on professional fees – this should be money well spent.
On large projects you might also need to move out while the work is done, so you will need to add for this. But if the work is to a house that has been empty for 2 years then VAT can be 5%, so this could be a saving. Make sure you discuss all this with your architect.
How To Calculate Build Cost
To get beyond the ball park figures we can to look at calculating the extension build cost. Each project is different, but at the early stage you can look at some simple factors that can give an idea of this cost. These will include the location and type of house, as well as the size and features of the planned extension. Each of these will put a slightly different spin on the ball park costs that we have looked at.
You can use the Extension Cost Calculator tool below to get a rough instant estimate of cost based on a few simple details. If you have any questions we will be happy to give you our advice.
Extension Cost Calculator
How To Control Extension Costs
If you have used the Extension Cost Calculator Tool, then you can see that there are lots of factors that affect the cost. While this is a simple tool, the reality is that this is just the tip of the iceberg. A huge number of things affect Build Costs, from the site, to the design and spec, and the chosen builder.
The skill in keeping an extension on budget is to know how to control these costs. Site Costs tend to be integral to the work and so are quite hard to bring down. Build Costs are big decisions such as the size and type of extension and will affect the cost a lot. Extra Costs are the easiest to control and can make a big impact.
Typical House Extension Costs
Typical extension costs will vary from about £1,200 per m² for simple work, up to £3,000 per m² for work that is complex. Basement work, glass extensions and bespoke design can be more.
The build cost is built up from a big number of elements. These range from the builder’s site set up costs, to the structure and foundations, to the building services and finish.
Typical Single Storey Extension Cost Breakdown
Each part of the work is needed, but how the cost is split between them will affect the budget. So if expensive finishes are put in the spec it will push costs up unless they can come down in other areas. The same is true for the choice of lights, floors, tiles and so on. This is where you need to work with a good architect to guide you through the technical design stage.
Detailed Cost Estimates
Ballpark costs are fine for an early stage and to help set a budget, but as the work moves on it is good to have a more detailed estimate. This will help you to be able to make informed choices on all of the parts of the work.
An architect can give good advice on house extension costs. In addition, for work of £100k or more it is good to get a Quantity Surveyor to look at the detail of the costs. A QS will look at the costs of hundreds of elements of the work. This detailed cost can then be turned in to a Schedule of Works which can be used to tender the work.
A detailed estimate is a great tool when making choices for the spec because it allows you to see exactly what you can spend. This chart shows the actual costs for a £185k extension we completed recently.
Detailed Extension Cost Breakdown
Use A QS For Accurate Build Cost Estimates
We work with a QS on most of our projects with a value of £100k or more. In most cases a QS will end up saving our clients more money than they charge for their service, so it just makes sense.
It’s a really good idea to keep the estimate up to date as you go through the Technical Design stage. In the example above, our clients wanted some high spec sliding glass doors and roof lights. These were about £10k more than standard spec, so to keep to the £185k budget we cut back about £4k on the finishes and worked with the builder to save £6k on the prelims.
A QS should have their finger on the pulse of current prices. It’s good to make sure that costs are accurate by using an up to date Building Cost Service with records of all kinds of build costs, such as the BCIS. The BCIS is approved by the RICS and publish a detailed schedule of rates. There is more info on the BCIS on the RICS website. An alternate to the BCIS is the SPONS price book. This is a similar detailed schedule of costs and rates. You can get the latest copy of SPONS through the RIBA Bookshop. For any more info on these terms you might like to read our Glossary.
Understand Extension Costs
Pay The Right Extension Cost
By this stage you will hopefully have a good idea of how much your house extension should cost. You should also know what parts of the work will cost the most, and where you can make an impact on the cost most easily if you need to.
The point of all of this, of course, is to make sure that you get the extension you want and that you are happy with what you spend.
To get the right price from a builder it is important to tender with good, detailed drawings and spec. These should show the builder clearly exactly what they are to do. Use a good architect to guide you through this and keep some key points to keep in mind to get the right price.
- Cost Estimates are great during the design stages. But they are not what you want from a builder. You need clear, fixed costs for the work. This is the only way to have cost certainty on site. Good builders prefer this too.
- This is best done by following a proper tender process. Your architect can invite builders to tender for the work. Give them 4 weeks to price the work properly and open all the tenders at the same time.
- If you follow the NBS Guide To Tendering for your project this will make the rules of the tender clear to the builders. This helps to avoid any surprises.
If you follow these points you should be well on the way to getting the right cost for your house extension. Good luck with your project, and if you have any questions just get in touch, we will be happy to help.