Easy way of calculating house extension costs?

telly-addict

Is there a simple, rough and ready way of estimating what a house extension would cost? I am aware that not all work would be the same however we would just like a quick, rule of thumb method to see whether our quote is way off the mark.
We will be getting other quotes from more builders. Another thread mentioned £100 per square foot - is that realistic for South East England? And is two-storey twice as expensive as single?

We are planning to extend out 2.5m, 2.5m wide, two storey onto a modern house. Thanks

brasso

Builders apply a formula, usually varying between £1000 and £1200 per square metre.

So if your extension is 2.5m x 2.5m, that makes a total of 6.6 sq metres which would come out at between £6250 and £7500. If it's 2 storeys, double it, so between £12500 and £15000.

This covers the structural work and the roof, but obviously not decoration, fixtures & fittings etc. You also have to factor in the cost of drawing up plans, making planning applications, possibly hiring an architect etc.

This sort of formula is certainly about right for a fair sized extension because I've had a few quotes for one recently.

peterdee_2

While the structural work is fairly straightforward to estimate (around £15,000 may be a good ballpark figure for your small extension), the fit-out costs will depend a lot more on your tastes - as its is possible to spend a significant amount of money on decoration.

Two storey extensions aren't usually quite twice as expensive as a single storey, although this will depend on the site conditions at your house (as the ground type will affect the foundations required).

There is some more info on house extension costs specifically in London and the South East here: http://www.architectureforlondon.co.uk/house-extension-cost.html

I would also advise getting lots of quotes from building companies to be sure that your builder isn't overcharging, good luck!

chappers

peterdee wrote: »

While the structural work is fairly straightforward to estimate (around £15,000 may be a good ballpark figure for your small extension), the fit-out costs will depend a lot more on your tastes - as its is possible to spend a significant amount of money on decoration.

Two storey extensions aren't usually quite twice as expensive as a single storey, although this will depend on the site conditions at your house (as the ground type will affect the foundations required).

But very small extensions don't usually work out using this formula anything under for anything under 10sq single storey or 20 two storey I would add 20-25% for the sq m price so more like £1250-£1500

mattj74

Does the rough £1000/m2 include pipes/wires for enabling electricity and gas heating (so only the actual light fixtures, radiators would cost extra to put in)?

donmaico

I am thinking about having a 4x8(approx) extension with bifold doors. piutched roof with sunlights and a kitchen(Wickes) to one side so that I would have kitchen diner.How much per sq mtr should I expect to pay including flooring and all services? thanks

marcg

How does a volume rather than floor area calculator help with a £ per sqm figure?

I would guess at £40-50k for the 4 x 8 extension, all in. So - build, kitchen units and appliances, architect, engineer, planning and building regs.

Though, obviously that figure could go up if you want fancy stuff in the kitchen or limestone flooring or polished plaster or or or....

Not likely to come down much though unless you are in Liverpool/South Shields/South West Wales.

donmaico

tez_c wrote: »

Our clients quite often ask us the same question, and it really depends what kind of extension - it's also worth factoring where abouts in the country you are as parts of the country can differ in price. We do house extensions in South Manchester and house extensions in Cheshire and we use our own software that automtically calculates but this online one is similar to what we use. http://www.planningni.gov.uk/index/tools/volume_calculator.htm

well i live in the south east , not far from London.We really are quite undecided at the moment and are looking at options.Obviously cost is a major factor but so is any possible return on our investment.
The house has a very small kitchen and a downstairs bathroom which we thought of turning into a wet room as we also have a bathroom upstairs.The kitchen could then open out into either a 4 seasons conservatory or an extension with large windows and bifold doors to allow as much light in as possible.

Using that tool i worked out the area to be 80-85 cu mtrs

donmaico

marcg wrote: »

How does a volume rather than floor area calculator help with a £ per sqm figure?

I would guess at £40-50k for the 4 x 8 extension, all in. So - build, kitchen units and appliances, architect, engineer, planning and building regs.

Though, obviously that figure could go up if you want fancy stuff in the kitchen or limestone flooring or polished plaster or or or....

Not likely to come down much though unless you are in Liverpool/South Shields/South West Wales.

yes we kind of thought between 40-60k all in .Kitchen wouldnt be too fancy, just Wickes. Ditto flooring. But I would want any conservatory built using good quality materials eg thermal glass that absorbs heat in the winter but keeps it out in the summer.I would also have to decide whether to have a polycarbonate ,glass or conventional roof

marcg

Glass roof will make getting building regs harder unless you replace the boiler or the new room is separate from the rest of the house (ie has a door) - too much heat loss through too much glazing.

Don't get plastic (polycarbonate) since it make a racket when it rains, lets through vastly less light (despite the manufacturers claims) and looks generally cheap.

In terms of thermal glass. Standard is £50 per sqm (1.7 U-value - the lower the more insulative) going up to £80 per sqm for 1.3 U-value. Bear that in mind when you get quotes - builders often think "special glass" = special price. On a 8m run of glass x 2m high the difference is only £400-500.

donmaico

marcg wrote: »

Glass roof will make getting building regs harder unless you replace the boiler or the new room is separate from the rest of the house (ie has a door) - too much heat loss through too much glazing.

Don't get plastic (polycarbonate) since it make a racket when it rains, lets through vastly less light (despite the manufacturers claims) and looks generally cheap.

In terms of thermal glass. Standard is £50 per sqm (1.7 U-value - the lower the more insulative) going up to £80 per sqm for 1.3 U-value. Bear that in mind when you get quotes - builders often think "special glass" = special price. On a 8m run of glass x 2m high the difference is only £400-500.

I did wonder about the heat loss aspect which I assume would make a house such as this much more expensive to keep warm given its open plan
http://images.google.co.uk/imgres?imgurl=http://www.conservatorysales.com/con%2520pics/shaw%2520kitchen%2520shot.JPG&imgrefurl=http://www.conservatorysales.com/Project%25201.html&usg=__ePyepUPB2VhX6Ze2TfPCJJ6OL74=&h=225&w=300&sz=22&hl=en&start=13&um=1&itbs=1&tbnid=rVYO3XiTZEe-XM:&tbnh=87&tbnw=116&prev=/images%3Fq%3Dkitchen%2Bconservatories%26um%3D1%26hl%3Den%26cr%3DcountryUK%257CcountryGB%26client%3Dsafari%26sa%3DX%26rls%3Den%26tbs%3Disch:1

maybe a standard roof with sunlights might be a better idea