Extension Cost Guide 2019
How much does an extension cost? This comprehensive extension cost guide will give detailed answers for all types of extension projects. An extension should be the best investment you ever make, and add value to your home, so keep up to date with 2019 extension costs here.
Instant Extension Cost Calculator
Get An Instant Estimate Online In Just A Few Seconds2019 Extension Cost Calculator
Cost Of Building An Extension
You can think of the total cost of building an extension in 3 steps. In this post we will take you through each step, with details and examples, so that you can see how much your extension will cost to build.
How To Set A Budget For Your Extension
A good first step is to work out a budget based on average costs per m2 to build an extension that is similar to yours. This is a simple and quick way to set a budget for your extension.
How Much Does It Cost To Build An Extension Per m²?
|Average Cost To Build An Extension Per m²|
|Basic Spec||Medium Spec||High Spec|
|Single Storey Extension||£1,250||£1,850||£2,500|
|Double Storey Extension||£1,050||£1,650||£2,250|
The figures shown in this table are 2019 average construction costs for London. Other parts of the country could be around 10 – 15% less depending on the location and site condition.
2019 London Extension Costs Per m²
The average house extension cost in London in 2019 is between £1,600 – £2,100 per m²
The cost of a typical high spec glass extension is between £2,300 – £2,700 per m²
Basement extensions cost on average between £3,500 – £5,000 per m²
Fine Tune Your Extension Cost Budget
The type of extension you are planning will have a big effect on the cost. For example, the cost of a single storey extension per m2 can be 20% more than a two storey extension. And a high spec frameless glass extension can be double the cost of a basic spec.
There is a balance between size, quality, and the value that the extension will add to your house. So you might need to fine tune your budget a bit, to set this balance at a point that is right for you.
Small extensions, under 10m2 or so, cost more as the site set up still has to be charged by the builder.
+ 25% TO BUDGET
Kitchen Extensions have more plumbing and electrics, and the kitchen itself, so cost a bit more.
+ 20% TO BUDGET
City Centre Extensions
City centre sites have parking and delivery restrictions, so extension costs are a bit higher.
+ 5% TO BUDGET
Access can be tricky on the side of a house, and there are more party wall issues to deal with.
+ 10% TO BUDGET
Conservation Area Extensions
It costs more to extend a house in a Conservation Area because of the type of materials you can use.
+ 15% TO BUDGET
Listed Building Extensions
The cost of extending a listed building can be very high due to the specialist nature of the work.
+ 30% TO BUDGET
The cost of a glass extension depends a large amount on the type of glass and frames used.
+ 100% TO BUDGET IF FRAMELESS
The main costs are in supporting the house, so it is far less to build under a garden.
+ 50% TO BUDGET IF UNDER HOUSE
Calculate Your House Extension Budget
Most projects include a few different types of work, and you will need to work out the costs for each bit to calculate your overall budget. The example below is typical, and includes side and rear extensions, and some refurbishment.
Two Storey Side Extension 20m2
Utility room with bedroom above; medium spec
Single Storey Rear Extension 15m2
Including new kitchen and glass doors; high spec
Internal Refurbishment 10m2
Painting and decorating; medium spec
To work out a budget for the project you can take average costs from the table above, add any fine tuning necessary, and calculate the budget as follows:
EXTENSION CONSTRUCTION COST
Two Storey Side Extension Cost
20m2 x £1,650 per m2
+ 10% fine tuning
Single Storey Rear Extension Cost
15m2 x £2,500 per m2
+ 20% fine tuning
Internal Refurbishment Cost
10m2 x £1,250 per m2
Total Extension Cost £93,800
SET A CONSTRUCTION COST BUDGET OF £90,000 – £95,000
Other Costs To Add To Your Budget
There are a number of other costs that you need to add on top of the Construction Cost Budget. The biggest of these is VAT which is normally 20%, but you will also need to set aside around 15% for other fees.
This graph gives an idea of the overall breakdown of costs for an extension project. Taking the example above with a Construction Cost Budget of £90,000 – £95,000 this would imply an overall project cost including VAT of around £150,000 – £160,000.
- CONSTRUCTION BUDGET 60%
- VAT 20%
- PROFESSIONAL FEES 15%
- PLANNING BUILDING REGS 2%
- PARTY WALL AGREEMENTS 3%
VAT On House Extensions
If the work is to a house that has been empty for 2 years then the VAT rate can be just 5%, so this could be a big 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 Extension Design Affects Cost
To get beyond the ballpark figures we can calculate the actual construction cost of an extension in more detail. Each project is different, but even 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.
How To Estimate House Extension Construction Costs
The construction cost of an extension is built up from a large number of elements. These range from the builder’s site set up works, foundations and structure, to the external envelope, plumbing, electrics and finish.
- Preliminaries cover the contractor’s site set up costs, excluding labour and materials, and are about 10%.
- Foundations and substructure (below ground works) are usually about 15% for a single storey extension.
- Structure including load bearing walls and steel beams makes up 30% of the cost of a typical extension.
- External Envelope including doors, windows, glazing and roof finishes are 15% for an average extension.
- Plumbing and Electrics including lights and heating will be around 25% of a typical house extension.
- Finishes are around 20% of the cost on average, but this is one area that can try to save money if needed.
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 Extension Cost Estimates
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 much more detailed cost estimate. This will help you to be able to make informed choices on all of the parts of the work.
This chart shows the actual construction costs for one of our projects, a £185,000 house extension built in London in 2018. It goes in to detail and breaks the extension cost down in to more than 20 items.
How Individual Building Costs Affect The Cost Estimate
An architect can give good advice on building prices from their experience. A detailed estimate is a great tool when making design choices for the spec because it allows you to see exactly what you can spend on each part of the work, to keep to your budget.
The example above shows that there are some key areas where the detailed design choices have a large affect on the cost of the extension. In fact for this £185,000 extension, with just a few tweaks to the choices of glazed doors, underfloor heating and lights you can reduce the construction cost by 20%.
Balance Design Choices With Budget
It’s a good idea to keep the cost 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.
Fine Tune Your Extension Cost Estimate
Other areas to look at in detail on a typical extension that can have a huge impact on cost are:
- Kitchens (can cost £10,000 or £100,000 depending on the supplier and features)
- Glazed Doors (can be £15,000 or £50,000 for a set of sliding patio doors depending on the spec)
- Floor and Wall Finishes (from £30 per m2 to £300 per m2 depending on what you choose)
- Extra Features (like under floor heating, high spec lighting systems and ceiling speakers)
So you need to make design choices on all of these items, and see where these costs sit within your overall detailed extension cost estimate. Then you will be able to calculate an accurate estimate of the cost to build your extension.
Accurate Extension Cost Estimates Using BCIS / SPONS
The Building Cost Information Service (BCIS) or SPONS Price Book have detailed schedules of rates for building projects. Using their rates it is possible to estimate the cost of an extension, and to check what the average costs of detailed parts of the work should be. The rates that they publish are very detailed and are not easy to understand. However they can be a useful tool for architects and quantity surveyors to estimate extension costs.
How Builders Affect Cost
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. But for this to become a reality we now need to find a contractor to carry out the work, at the right price.
Get A Schedule Of Works For Contractors To Price
The detailed cost estimate that we looked at above, can now be turned in to a Schedule of Works which can be used to tender the work. The schedule is split down in to all of the key parts of the work, and you should now already have a good idea of what each bit is likely to cost. It is sent out blank (without any cost information) to contractors for them to price.
Run A Proper Tender Process
To get the right price from a builder it is important to tender with good, detailed drawings and spec. These should show the builder 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 you do not want an estimate from a contractor. We will repeat that. You do not want an estimate from a contractor. You need a clear fixed price for the work, because this is the only way to have cost certainty on site.
- Follow a proper tender process to get proper prices. Your architect can help to select and invite builders to tender for the work. Give them 4 weeks to price the work and open all the tenders at the same time. Always follow the NBS Guide To Tendering as this will make the rules of the tender clear.
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.