Cost is one of the biggest factors when deciding to extend your property.
It's also not the easiest question to answer as the cost can depend of a number of factors from ground conditions all the way through to finished decor.
So we're going to give you some guideline figures so you don't feel completely in the dark.
Firstly we need to dismiss the common old figure of £1,000 per m². This figure is well aged and in present day 2016 it's basically out of date. In most cases if you base your guideline price on this figure you will be under price by a considerable amount.
From our experience it would be more advisable to allow for £1500 per m².
£1500 per m² is now a more realistic figure to use as an estimate for the cost of an extension in 2016, and in our experience it's the most quoted figure from contractors and general builders when they initially meet with a potential client.
To give an example:
The most common basic single storey extension is generally around 3M X 4M
So 3m x 4m =
12 x 1500 = 18,000
£18,000 + VAT (20%)
Single storey extension = £21,600
What about a double storey extension?
How much does a two storey extension cost?
Let's take the same footprint, 3m x 4m.
On your single storey price you have already allowed for basic foundations and roof. These do not need to be reconsidered, you're going to need them whether you build a single storey or a double storey extension. So for the first floor we allow 60% of the cost of the ground floor.
A simple equation is to take the ground floor figure and multiple it by 1.6
So 18,000 x 1.6 = 28,800
£28,800 + VAT (20%)
Double storey extension = £34,560
The above examples would be a uncomplicated extension built to a standard specification with no significant structural issues or requirements.
Keep in mind small things can add up, as other factors are added into the construction the price per m² will rise. As a further guide if your build were to involve some common structural work, we would suggest you increase your £'s per m² to £1700 per m². Further more if you require higher quality materials or have a challenging structure or site conditions then we would suggest increasing your £'s per m² to £2100 per m².
Whilst considering your design also keep in mind that the biggest variance of cost will come from the individual specification. Individual specification is also the most common reason we see for an extension going over budget. So...
Do these guide prices include kitchens and bathrooms?
If you are looking to add either a kitchen or bathroom or even both, you will need to factor these costs in. Bathrooms and kitchen can be very personal to the choice of the client, so are extremely hard to put a price on.
You will also need to consider that with the addition of either you will need to allow for services such as additional plumbing and foul drainage.
Common features such as roof lanterns, additional glazing and bi-fold or sliding doors will all increase costs. For example a good quality bi-fold door would cost upwards of £1200 per linear metre.
Is location a factor?
Yes, if you are located in the South East you can expect to pay a little more than rest of the UK.
We suggest you pass your plans on to at least 3 recommended, reputable builders so you can obtain quotations for your build. Once you have chosen your builder, ensure you have a set contract. For works such as extensions, the FMB provide the 'Plain English Contract' and JCT also provide contracts you can use between you and your builder.
Lastly these figures are provided as a helpful guide for the costs of building work. They do not include professional design fees, local authority fees or VAT.
We hope this article helps as you embark on your building project.