Easy way of calculating house extension costs?

A question about : Easy way of calculating house extension costs?

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 Ј100 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

Best answers:

• 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.
• While the structural work is fairly straightforward to estimate (around Ј15,000 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: https://www.architectureforlondon.co....sion-cost.html
  I would also advise getting lots of quotes from building companies to be sure that your builder isn't overcharging, good luck!
• Does the rough Ј1000/m2 include pipes/wires for enabling electricity and gas heating (so only the actual light fixtures, radiators would cost extra to put in)?
• 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
• How does a volume rather than floor area calculator help with a Ј per sqm figure?
  I would guess at Ј40-50k 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.
• 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 Ј50 per sqm (1.7 U-value - the lower the more insulative) going up to Ј80 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 Ј400-500.
• Heck there is so much to think about.I just read that kitchen conservatories are no longer classified as such by local councils so building regs have to be adhered to which could mean a lot more expense and hassle .Also being opem thge house would use a lot more gas to heat it,i am now thinking in terms of either a small extension to increas the size of the kitchen with an adjacent conventional conservatory with doors leading from both the new kitchen and existing lounge.The other alternative would be a sun lounge with a solid pitched roof which would , as before include a kitchen
• Conservatories have lower heat retention building regs requirements than straight extensions. You can also get cheap upvc ones added a lot more easily than knocking down walls and sorting out flooring joins and other making good.
  Don't get me wrong, you can add a fully glazed extension, but it will need careful design - this is why I point out the boiler replacement as being key. A typical 15+ year old boiler will have 75% efficiency. New ones have 97% and this makes a massive difference to the before and after energy calculations. Often enough to offset the overglazing.
  The kitchen extension + cons is definitely the easier way to go from my architect's point of view but not necessarily the best solution for the house. The other thing to consider is the loss of light to the lounge. Even with a conservatory through the old window opening there will be much less light in the lounge. I recently completed a kitchen extension on a small terraced house that left the kitchen and lounge windows facing a small courtyard instead of infilling it. The existing house had the kitchen (and bedroom 3 above) projecting into the garden (think typical Victorian terrace) and the new extension was full width on the end of the kitchen. In this way there was no loss of light to the original house.
• I am buying a small victorian terraced house and need a new kitchen extension 3m * 3m my boyfriend plans to dig the footings and do the main work such as lay the concrete flooring etc. We really just need a builder to build the walls and put a roof on the extension as we have people to do the plumbing and electrics. We are hoping to do this for Ј5k do you think this will be possible?
  thanks
• Is the builder going to provide all materials and plaster walls/ceiling, install rainwater goods? What sort of roof will it be a flat felt covered boarded roof or pitched tiled roof? The type of door(s) and window(s) will greatly influence costs.
• We will provide the material for digging foundations and laying the flooring because my boyfriend is going to do that. We would need someone to do the rest. Flat roof on the top of it. We think it will be 3m * 3m any idea of the cost?
  thanks
• I just checked to see what I paid 2 yrs ago for my extn, but it's impossible to compare with what you propose. As your boyfriend is doing the foundations, I would suggest you approach 3 builders and ask if they could give you a quote for the work. You will have to be very specific what you want doing and what materials you specify, i.e. bricks, doors, windows. If you haven't already applied for building regulations you will need these, you may also need planning permission and if you happen to be in a conservation area, this will cause further problems, a flat roof extn may be a no no.
  Also remember that not all builders will want to "finish off" what someone else has started.
• I was told for the South East an all in price to include the windows, pitched roof, architrave and skirting etc. it is Ј1500/sq m for the downstairs and then Ј1200/sq m for the upstairs. This may also include a 'standard' ensuite too.
• High Im planing to have an extention built soon in south west scotland.
  Down stairs is 6m wide by 6m depth up stairs 6m wide by 3m depth glass sliding doors pitched roof with 6m wide and 3m depth patio area on top of lower extention with balustrade system in glass