How much for extension over garage?

Assuming that the garage has been built to withstand a storey being built on the top (will it have?), how much would I be looking at for an extension to a house by building over a garage?

Technically would be single storey but would make the build a double storey at the end.
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  • Doozergirl
    Doozergirl Posts: 33,716
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    When was the garage built?  You're assuming the foundations are okay, but that is the biggest consideration as the quality of that build determines whether it's suitable to be built on at all. 

    Even if we knew that, we don't even begin to know the size of the garage, the shape of the house, how it connects together, the spec of the new build.  

    Pricing isn't easy when you've got all the information you need, it's impossible when you have nothing to go on.  
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  • Section62
    Section62 Posts: 7,496
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    Assuming that the garage has been built to withstand a storey being built on the top (will it have?), how much would I be looking at for an extension to a house by building over a garage?
    If it is a single storey garage on the end of a house it is unlikely the walls were constructed to carry an additional storey.  If it is a 4" wall (as typical) then even assuming the foundations were adequate to carry the load, you'd need to come up with an inventive solution for building the new (1st floor and above) walls to modern standards for insulation/damp protection.

    It can be done, but may be cheaper to demolish the existing garage and start again.
    Technically would be single storey but would make the build a double storey at the end.
    If the new work is on top of the ground floor garage then it would count as double storey for planning purposes, even if you are only adding one storey.

    How old is the house/garage?
  • FreeBear
    FreeBear Posts: 14,253
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    Section62 said:
    Assuming that the garage has been built to withstand a storey being built on the top (will it have?), how much would I be looking at for an extension to a house by building over a garage?
    If it is a single storey garage on the end of a house it is unlikely the walls were constructed to carry an additional storey.  If it is a 4" wall (as typical) then even assuming the foundations were adequate to carry the load, you'd need to come up with an inventive solution for building the new (1st floor and above) walls to modern standards for insulation/damp protection.
    One house that I know of had a single storey extension built over an existing car port. The problem they faced was the party wall didn't have any foundations of note. For what ever reason, demolishing and rebuilding the wall was not an option. They went for piling and a steel framework to support the extension constructed from a mix of SIPs and in situ constructed stud walls. Bit of a slap-dash build if you ask me, but it got signed off, so must have satisfied Building Control.

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  • When was the garage built?  You're assuming the foundations are okay, but that is the biggest consideration as the quality of that build determines whether it's suitable to be built on at all. 

    Even if we knew that, we don't even begin to know the size of the garage, the shape of the house, how it connects together, the spec of the new build.  

    Pricing isn't easy when you've got all the information you need, it's impossible when you have nothing to go on.  
    Yes but come on, people can make educated guesses based on averages.

    I forgot this site needs absolute specifics to give any reasonable answer.

    A simple "many are about £10k on average" or a "when I had mine done, it was £50k" is what I'm looking for. Because the difference in the two is quite significant.

    You also could have said "if the foundations are okay, it'll be £15k, if not, closer to £30k" or something to actually guide instead of, what is effectively, a useless reply.
  • FreeBear
    FreeBear Posts: 14,253
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    When was the garage built?  You're assuming the foundations are okay, but that is the biggest consideration as the quality of that build determines whether it's suitable to be built on at all. 

    Even if we knew that, we don't even begin to know the size of the garage, the shape of the house, how it connects together, the spec of the new build.  

    Pricing isn't easy when you've got all the information you need, it's impossible when you have nothing to go on.  
    Yes but come on, people can make educated guesses based on averages.

    I forgot this site needs absolute specifics to give any reasonable answer.

    A simple "many are about £10k on average" or a "when I had mine done, it was £50k" is what I'm looking for. Because the difference in the two is quite significant.

    You also could have said "if the foundations are okay, it'll be £15k, if not, closer to £30k" or something to actually guide instead of, what is effectively, a useless reply.
    You failed to give even the most basic of information. i.e. Size.
    Plucking a figure out of my [redacted], £4,000 per square metre will get you a weather tight shell out side of London. If you want a pitched roof, add another £20K.

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  • EssexExile
    EssexExile Posts: 6,093
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    edited 19 November 2023 at 2:49PM
    £45,000 about 18 years ago.

    Edit: I forgot the underpinning because the foundations weren't good enough - add another £1500.
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  • I forgot this site needs absolute specifics to give any reasonable answer.

    A simple "many are about £10k on average" or a "when I had mine done, it was £50k" is what I'm looking for. Because the difference in the two is quite significant.

    You also could have said "if the foundations are okay, it'll be £15k, if not, closer to £30k" or something to actually guide instead of, what is effectively, a useless reply.
    TractorFactor - forum member 9 months - post count 63
    Doozergirl - forum member 222 months (25x as long) - post count 33,597 (533x)

    I'm not sure if you've had a good enough read around these forums to realise Doozergirl is a pretty experienced property developer who has an immense amount of knowledge & experience and as they say "been around the block". Heck, she's even been to the infamous New Year's Eve party during lockdown  :D

    As she says, it is nigh on impossible to give even rough figures without further information about the site, existing build, soil type, drainage qualities, conservation area, availability of local trades etc etc. My own experience includes several property developments over 25 years with typical spends of £250-£700k. I wouldn't be able to guess without loads of photos/map and/or a site visit.

    Is this a useless reply too?

    PS Freebear has given a ballpark figure that might work. Unless you're on a slope, old mine, flood plain etc

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  • Doozergirl
    Doozergirl Posts: 33,716
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    edited 19 November 2023 at 10:44PM
    I forgot this site needs absolute specifics to give any reasonable answer.

    A simple "many are about £10k on average" or a "when I had mine done, it was £50k" is what I'm looking for. Because the difference in the two is quite significant.

    You also could have said "if the foundations are okay, it'll be £15k, if not, closer to £30k" or something to actually guide instead of, what is effectively, a useless reply.
    TractorFactor - forum member 9 months - post count 63
    Doozergirl - forum member 222 months (25x as long) - post count 33,597 (533x)

    I'm not sure if you've had a good enough read around these forums to realise Doozergirl is a pretty experienced property developer who has an immense amount of knowledge & experience and as they say "been around the block". Heck, she's even been to the infamous New Year's Eve party during lockdown  :D

    As she says, it is nigh on impossible to give even rough figures without further information about the site, existing build, soil type, drainage qualities, conservation area, availability of local trades etc etc. My own experience includes several property developments over 25 years with typical spends of £250-£700k. I wouldn't be able to guess without loads of photos/map and/or a site visit.

    Is this a useless reply too?

    PS Freebear has given a ballpark figure that might work. Unless you're on a slope, old mine, flood plain etc

    Aw, that party!  I remember exactly where I was - physically and imaginatively.  I just searched for a random liqueur on the forum (galliano) and found the thread straight away ☺️ 

    For tractor person, we may not need specifics but an idea of anything at all is a good start.  I'd like to know what you do for a living if customers being entirely vague with you enables you to get anywhere close to providing the right sort of advice to them. 
    Everything that is supposed to be in heaven is already here on earth.
  • Whilst the foundations may be okay, are the external walls double skinned?
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  • Bigphil1474
    Bigphil1474 Posts: 2,279
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    If you work on around £3k per square metre you won't be far off. Extra cost if there are any issues. Add 10% just in case, and have a bit spare just in case just in case. 
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