Building an Extension Should Add Value to Your House
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 costs needs to be right.
We want all our clients to get the best value for money. A great home extension is key and will add value to the house. But how do you know what you should pay for the work? In this post we will give you the answers so you can be sure you will get the right price.
How To Get Value For Money On An Extension
- Get a Cost Estimate and Set a Budget.
First of all, look at the Average Cost of Building an Extension. Then get an estimate for extending your home using our free Extension Cost Calculator. This will give you all the the info you need to set a budget.
- Update the Budget As You Make Design Choices.
To keep on top of the budget you need to know How Design Choices Affect Cost. You’ll want a Detailed Cost Breakdown so you can use it to Reduce Costs to hit your budget.
- Get the Right Builder for the Right Price.
A fixed quote from a good builder is worth its weight in gold. The key is to tender with good drawings and spec to Get the Right Price. And you need to know How A Builder Can Affect Cost.
House Extension Costs in London 2018
What is the cost to build an extension? It will depend on the type, size, and specification. This table shows average construction prices per m2. These are London figures – in other parts of the UK they could be up to 20% less.
|Average Build Prices per m²|
|(2018 in London)|
|Basic Spec||Medium Spec||High Spec|
|Single Storey Extension||£1,250||£1,850||£2,500|
|Double Storey Extension||£1,050||£1,650||£2,250|
A First Cost Guide
A First Estimate Should Be A Guide Price Per m²
To work out a first estimate, think in terms of the size of the extension in square metres. Use the table above to get the right figures, and take off around 20% if you are not in London or the South East.
For instance, you might build a 20m2 medium spec extension at £1,850 per m2, plus 10m2 refurbishment at £1,250 per m2.
So, you could expect costs of £37,000 for the extension, and £12,500 for the refurbishment. Giving a grand total of £49,500 for the work.
Prices Per m² Change Each Year
Don’t make the mistake of referring to out of date websites or prices. Be sure to use up to date prices to estimate the cost of building an extension.
We keep our site up to date and all the figures in this post are for 2018. This is really important – prices have gone up nearly 15% in 2016 and 2017.
2018 Construction Prices Per m²
- The average house extension cost in London is £1,250 – £1,850 per m².
- For a glass extension or high spec design £2,000 – £2,500 per m² is more likely.
- Basement extensions have the highest costs at more than £3000 per m².
Average Extension Prices
Of course, there are lots of types of extension and this will affect the price. The cost of a single storey extension per m2 will be a bit more than for a home extension built over two storeys. A flat roof could cost less than a pitched roof. Your ideas for new flooring, windows, kitchens, bathrooms, plumbing, central heating, insulation, will all affect the quotes for your new extension.
But remember the extra space will add lots of value to your home. So a price guide is really useful to let you make the right choices and save you money.
Specification And Building Materials Matter More Than Size
The size of an extension can add or take away around 15% to the average price per m². But the level of specification has a much bigger effect than size. Look back at the table above and you’ll see that a high spec extension can add more than 50% to the price of a medium spec.
Construction Prices by Extension Type
What Should You Pay For A Small Extension?
Small extensions can cost a bit more per square metre. This is because all of the site set up work still has to be charged by the builder. So if an extension is less than about 10m2 it is likely to cost 15% more or so.
Average construction prices for a small extension are £1,450 – £2,875 per m2.
How Much Is A Single Storey Extension?
Single storey extensions are a great way to add value to a home. The average cost of a single storey extension is far less than the house value per square metre.
Average single storey extensions cost £1,250 – £2,500 per m2.
How Much Is A Two Storey Extension in 2018?
Ground floor foundations only need to be paid for once, so the price to build a double storey extension per m2 is less. They are a great choice to add value.
Average two storey extensions are £1,050 – £2,250 per m2.
What Is The Cost of A Glass Extension?
The cost of a glass extension will depend a large amount on the type of glass.
The average cost to build a glass extension with glazed units and metal structure is £2,000 – £2,500 per m2.
But glass extensions in frameless structural glass can be £5,000 + per m2.
How Much Is A Basement Extension?
The main costs to build a basement are in the dig out and supporting the house above. So, for instance to build a basement under a garden will be far less than to build it under a house.
On average basement extensions are £2,100 – £4,200 per m2.
How Much Is A Side Extension?
The price of a side extension is similar to a single storey rear extension. But it can be less easy to build on the side of a house, and a bit more work can be needed to deal with any party walls.
So average side extensions are £1,375 – £2,750 per m2.
How Much Is A Kitchen Extension?
Most extension projects include for a new kitchen. So, the price of a kitchen is included in the average costs per square metre above.
Kitchen costs can vary from £5,000 for a simple off the shelf design, to £50,000 + for a bespoke kitchen design.
Extension Cost Calculator
In less than 2 minutes you could get an estimate of cost to build your extension. The calculated build cost estimate is displayed just below the questions. And if you have any questions or want to discuss your project with us please just ask.
Get Your Estimate Now
How To Set A Budget For Your Extension
A good architect will be able to help to show anything that could affect the cost of building an extension. They can also help to make clear what needs to be included in the budget, and what does not. For example, if you add a kitchen or new bathroom this can change the costs a bit. But you could decide to fit these later to save money.
It’s normal for building costs to be given ex VAT (as VAT can vary) so you will need to factor that in too. Currently VAT is 20% on most building work, but there are VAT reductions to 5% or even 0% in some circumstances. You also need to add on for Planning Application fees, Building Regulations and Party Wall awards. Typically if you set aside around 15% of your budget to cover professional fees that will be about right.
House 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 Permission, Building Regulations and a Party Wall Agreement. The last 15% is on professional fees – architect, structural engineer, project manager – 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.
VAT Rate For Extensions
If the work is to a house that has been empty for 2 years then VAT can be just 5%, so this could be a saving. There are even times when VAT can be 0% but this usually relates to new build work and not extensions. Make sure you discuss all this with your architect.
How Design Choices Affect Cost
To get beyond the ball park figures we can to look at calculating the actual construction cost of the extension in more detail. Each project is different, but at the early stage you can look at some simple factors that can give an idea of how this will break down. These will include the location and type of house, as well as the size or floor area, 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.
Typical Construction Prices For Extensions
Typical building prices 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 construction cost is built up from a big number of elements. These range from the builder’s site set up works, to the structure and foundations, to the building services and finish. These can be split in to 6 main types as shown in the chart below.
6 Types of Building Costs
Preliminaries covers the contractor’s site costs, excluding labour and materials, and are about 10%. Foundations are usually 15% and Structure makes up around 30% of the overall. The External Envelope including doors, windows, roof finishes can be 15% or more. Plumbing and Electrics is around 25%. Finishes are high at around 20% but this is one area that can try to save money if needed.
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.
How Much Do Extensions Usually Cost?
You can use the Instant Cost Calculator tool to get a rough instant estimate based on a few simple details. This should give you a good estimate cost of an extension. If you have any questions we will be happy to give you our advice.
Ballpark figures 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.
Architect’s Estimate or Quantity Surveyor
An architect can give good advice on building prices. In addition, for bigger extensions it is good to get a Quantity Surveyor to look at the detail. Quantity Surveyors will look at the costs of hundreds of elements of the work. This detailed cost breakdown can then be turned in to a Schedule of Works which can be used to price 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 costs for a £185,000 + VAT extension we built in London in 2017. It goes in to more detail than the 6 types of building costs described above, and breaks the figures down in to more than 20 items.
Detailed Breakdown of Construction Prices
How To Reduce Costs
If you have used the 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.
Make Savings Where You Can
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.
Get The Right Price
Make Sure Your Estimate Stays Accurate
We work with a Quantity Surveyor 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.
Balance Design Choices with the Budget
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.
BCIS and SPONS Building Price Books
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 Price Service with records of all kinds of building prices. For example the BCIS is an approved guide that publishes a detailed schedule of rates. There is more info on the BCIS on the RICS website. An alternate 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.
How A Builder Can Affect Cost
Pay The Right Amount For Your Extension
By this stage you will hopefully have a good idea of how much your house extension should be. 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.
Tender Properly To Get The Right Price
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 need 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.
- Estimates are great during the design stages. But they are not what you want from a builder. You need a clear, fixed price for the work. This is the only way to have cost certainty on site. Good builders prefer this too.
- Follow a proper tender process to get proper prices. Your architect can invite builders to tender for the work. Give them 4 weeks to price the work and open all the tenders at the same time.
- If in doubt follow the NBS Guide To Tendering. This will make the rules of the tender clear to the builders and avoid any surprises.
If you follow these points you should be well on the way to getting the right price 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.