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  • How much do you reckon this extension would cost? AIBU?
  • Premier Icon thisisnotaspoon
    Full Member

    So we’re planning to do some (fairly major) work on the house. It’s currently a fairly typical 1970’s detached with a dining room and a kitchen (3.25m x 4m each) at the back, and a lean to conservatory (~6.5m x 2.5m)across the whole of the back.

    Plan is to remove the internal and external walls and replace the conservatory with an extension with the obligatory bifold doors and velux windows.

    We’ve arranged for 3 quotes, the first two are all in one companies that specialise in this sort of thing, one has their own SIP system, 2nd is more conventional, the 3rd is just a builder.

    1st quote just came back at £45k to a plastered finish, so no flooring, kitchen, decorating included! That seems a lot (double!) based on various extensions cost such and such per square meter, going open plan + steel lintles cost such and such etc guides we’d budgeted based on. Especially as that was supposed to be the cheap option that doesn’t require proper foundations!

    Premier Icon Cougar
    Full Member

    What do your other two quotes say?

    Premier Icon trail_rat
    Free Member

    I did similar in summer with a kitchen and was 35k no labour….. That includes the overpriced drawings I was told I had.

    My neighbour was quoted 55k no kitchen

    So your numbers don’t seem.too out there.

    Premier Icon trail_rat
    Free Member

    Fyi we didn’t go with bi-folds – cost increase was mental just a French door and big window for about 1/4 the price of an equivalent bifold

    We only had one wall removed.

    If you think your do all what you list for 20k that’s unlikely.

    Premier Icon ayjaydoubleyou
    Free Member

    The price is predominantly labour (maybe not with the SIPS but they price to match or just undercut conventional construction).
    Builders are rushed off their feet at the moment, so there will be some supply and demand going on.
    If you have some patience (and no urgent deadline) and have some amount of DIY ability then organising the various trades yourself MAY come out cheaper than getting an all in one.

    Premier Icon Spud
    Full Member

    Our build in 2015 was around 60k 8x4m extension after removing the old smaller extension, but this did mean the additional footings were limited in size. We had other work done at the same time (new bathroom, boiler went pop too) So some of the same guys did more work on the back of it for us. That was ready to live in, kitchen, tiled throughout, underfloor heating, decorated and 2x 3m sets of alu bifolds.

    EDIT: I did a fair amount of shopping round for things like the bifolds, and we paid each trade the builder engaged directly in the main too.

    Premier Icon mrmonkfinger
    Free Member

    Fancy doors.

    Holding up existing house with some big steel.

    20 ish sq m of extension?

    Seems reasonable.

    Premier Icon jaminb
    Full Member

    we had similar size extension and like for like it was c.£60k. But that was 10 years ago, down south and very deep footings!

    Premier Icon wobbliscott
    Full Member

    Our build in 2015 was around 60k 8x4m extension

    Similar to mine. Build costs were £26k for a finished empty shell but we increased scope to replace French doors for two bifold (+£1400), install UFH and upgrade CH system (+£2k). Didn’t include kitchen but did include labour to fit kitchen. Total costs including flooring, furnishing, big telly and upgrading garage door to electric sectional door etc came in at £50k, though the upgrades were funded mostly by taking out the extortionate cost of the engineered wood floor and going for Laminate instead keeping us within budget with all the upgrades.

    Premier Icon perchypanther
    Free Member

    £38,489.16 plus VAT.

    Do I win a prize?

    Premier Icon thisisnotaspoon
    Full Member

    What do your other two quotes say?

    They’re coming round this week.

    If you think your do all what you list for 20k that’s unlikely.

    Who said £20k? We’d sort of assumed:

    6*2.5m = 18m2 = £18000 for the extension (based on it being just a bare shell so at the very bottom end of the £1k-£2k depending on finish range).

    3 x £2k for the steel beams + £2k to demolish the inside wall (based on https://www.myjobquote.co.uk/costs/installing-rsj), there isn’t really an outside wall as the old dining room has one big window over the whole wall, and the kitchen is the same but with 2x windows either side of a door, so more like replacing existing concrete lintels with steel. +£2k for electrics and plastering.

    So ~£28k was the starting point in our head, possibly less for the 1st quote as I’d assumed they’d just re-use the concrete pad of the existing conservatory (they did suggest just re-roofing it and doing the steelwork which is possibly the cheap option if the others are higher again).

    ~£60k with kitchen and flooring is getting on for sell it and just buy a bigger/nicer house territory.

    Builders are rushed off their feet at the moment, so there will be some supply and demand going on.

    The few that we’ve spoken to said it had been steady. They had a break over Christmas and stopped for the 1st lockdown, but since then it’s been constant but lead times still only a month or two.

    Premier Icon ebikegum
    Full Member

    OP – sounds very much like the work that we are currently nearing completion on.

    We’ve had a knackered old timber conservatory removed and new ‘proper’ room built in its place with previous dining and kitchen internal walls removed to create the de-rigueur large open plan living area. New room is approx 4m x 4m with alu bi-folds out to a new decked area and new porch on the front of the house. New Windows throughout, Oh, and a new kitchen too.

    As for cost, exc kitchen, we are at about £110,000 plus the dreaded VAT. I expect that we will be at £175k inc vat once all finished. Admittedly, we have gone for a few nice finishes, but on the whole, nothing outlandish by any means.

    You don’t get a lot for sub £50k these days unfortunately.

    Premier Icon alanl
    Free Member

    I worked on a quality extension,5 years ago, around the same size and idea as yours, but with a divider down the middle to keep the kitchen separate. 2 Velux, patios doors on one side, and normal composite door on the other side. It looked stunning when finished. The Builder said he hadnt charged enough at £35k, as he reckoned he had only just covered wages.

    Premier Icon trail_rat
    Free Member

    Who said £20k? We’d sort of assumed:

    i assume that 45k being double what you expected was a typo ?

    1st quote just came back at £45k to a plastered finish, so no flooring, kitchen, decorating included! That seems a lot (double!)

    i also assumed you were installing 1 big beam across the back and T’ing off with the perpendicular beam – which was gonna be some big steels to cope with point load and potentially even vertical bracing at the ends pending what your buildings made of

    As i said at the start our materials*for what i think is a lesser job were more than your ball park expectation budgeting for the works – and thats with shopping around – nothing flash.

    The shell and steel works alone cost 25k in materials – although the steels were a canceled order from one of my old mans previous jobs because someone changed their mind to the size of the opening after materials were ordered – I got them for scrap value.

    i was fully aware that at client prices i was better moving house . i dont think anyone extends for convenience do they – it almost always costs more than moving to an equivalent house.

    *Edit sorry that’s a lie..I also added in a new oil tank for that and I laid a concrete pad for my new shed due to having excess rebar concrete dpc and digger on site

    Premier Icon db
    Full Member

    Seems reasonable, our conservatory was over £50k.

    Premier Icon jim25
    Free Member

    Where are you?
    I’m a builder,London based and for quick guide prices I would quote at £3,000 per square metre
    6.5 x 2.5 + 16.25m = £49k

    Plus, what builder told you you can reuse you conservatory slab/footings…! Big no no there. No way your conservatory footings will be adequate for building regs sign off and the floor will likely have zero insulation in it, again not going to be passed off by building control. That’s bad information

    Premier Icon scamperjenkins
    Free Member

    Just embarking on this. Want to replace our kitchen and knock through into the utility/dining room (old garage) to form small kitchen diner. Then I want to knock through the other side of utility to extend the kitchen diner and also build a small garage with internal access (other option is not to knock wall down but have a door only into garage and a seperate utility/drying room/shower room. This outside space is 7x3m. Have £40k to play with – not so sure if that is nearly enough.

    Premier Icon ollie_the_brave
    Free Member

    I’m a builder. What you’re asking is along the lines of ‘how long is a piece of string’.

    For a bog standard build, up to a non plastered shell with a site with easy access, local to me (Manchester) I’d be advising clients to be budgeting in the region of £1,650 + VAT per sq m of additional floor space. On top of that you’ll need all doors, windows, electrics, plumbing, floor coverings, second fix joinery (skirting, architrave, internal doors) plastering etc etc.

    Also, SIP structure DOES need ‘proper’ foundations. The design of such may be different to brick and block, but still needs to be structurally sound and proven.

    Also II 6.5m bifolds will be at the very limit of functionality at that length. You’ll only open them one or two days a year fully. Have a look at sliding options, you’ll save money and have the same/similar effect with the bonus of being able to either open them fully, partially or just 6″, as well as being able to put furniture up to them.

    Premier Icon scamperjenkins
    Free Member

    Ollie, am I stating the obvious that when quoting, builders like to see architects drawings rather than vague client waffle?

    Premier Icon johndoh
    Free Member

    We did the extension on this house back in 2008/09 – removing an internal (load bearing) and external wall to put the extension in. We also had a downstairs loo installed at the same time. The windows / doors were custom built solid oak and it cost us £25k at mates rates (ie no VAT) so I don’t think your price of £45k is far off the mark as there will be some structural/groundwork going on which isn’t cheap.

    Premier Icon ollie_the_brave
    Free Member

    Ollie, am I stating the obvious that when quoting, builders like to see architects drawings rather than vague client waffle?

    Yep. Complete waste of both parties time, otherwise.

    No point even putting a quote together until the building regs drawings and specs are available and approved, as it won’t be correct.

    Architect may be designing something to be build specifically and structural engineers need to specify the design (in this case the steelwork will be the major consideration).

    Experience taught us that clients tend to get pissed off when what they consider was your firm quote changes due to design changes or a differing structural design to your assumed one! A lot of folk have no concept of how much building work costs, especially the bits underground, and think it is way cheaper than it is.

    Premier Icon Tallpaul
    Full Member

    Based on what we paid for a 15m2 extension 3 years ago, with inflation, the extra size and knocking down the internal dividing wall, my guestimate was ~£40k. If £45k includes the daft bi-folds, it doesn’t sound that expensive…

    Premier Icon mrmonkfinger
    Free Member

    A lot of folk have no concept

    Could have stopped there.

    Premier Icon 5lab
    Free Member

    out of interest (as there’s a couple of builders on here, clearly) – if you wanted to do a 2 story extension instead of one (and a pitched roof, etc), is it just a case of double it? I guess some of the work is easier (one set of footings, etc), but some is harder (more scaffolding to put up, adapting the existing roof, etc etc)?

    Premier Icon perchypanther
    Free Member

    is it just a case of double it?

    No. Not really.

    It’s a bit more complicated than that.

    In general terms it’ll reduce your cost per m2 of gross floor area.

    Premier Icon VanHalen
    Full Member

    you never get much extension wise for less than £40k if you pay someone to do it. removing extra internal walls will bump up costs.

    allow at least 20% contingency to that price for all the extra stuff they need to do/find during the works.

    If you have a architect trying to get their ‘vision’ implemented you can double the price. at least.

    Premier Icon ollie_the_brave
    Free Member

    is it just a case of double it?

    No. Not really.

    It’s a bit more complicated than that.

    Yep. It’s a good starting point to give an overall idea but is pretty ballpark – each project is different. Throw a tricky roof, tricky scaffold, lots of rooflights, lack of delivery/waste removal access etc etc into the mix and it soon increases.

    Premier Icon trail_rat
    Free Member

    If you have a architect trying to get their ‘vision’ implemented you can double the price. at least

    The wrong architect maybe.

    I got exactly what I wanted but j had my own vision and I knew what the architects vision which looked nice would cost over and above mine. People easy led astray by the pretty pictures that’s his job

    Premier Icon joebristol
    Full Member

    We had an extension that was finished Feb 2019. Knocked a conservatory down at the back – those foundations weren’t any good for a proper structure and wouldn’t have met building regs. We ended up with a room 4.5mx5.5m I think – although it also went down the side of the house and fitted in a small wet room. Would guess that area and a bit of extra hallway is 1.5mx5m. We built on concrete pads with steel reinforced concrete beams so we could easily build right up to the boundary. Single story only. A frame kind of roof, 2 x velux windows, 3.5m long bifold doors, massive amount of insulation to hit the R value required given a lot of glass. We built timber skin inside with block outside as it meant it took up less room down the side of the house where space was tight.

    Came to something like £85k and that was with a mate doing it and I don’t think he made as much as he’d have liked. Surprising how things add up very quickly.

    I shopped around for the wet room materials / bi fold doors / new front door / windows / flooring etc. I also did all the decorating.

    Premier Icon squealer
    Free Member

    I’m very glad I don’t live in the south east with prices like £3,000 per sq m!

    This probably won’t help but I put an extension on the back of a rental house we bought which was 8.5m x 6.5m plus a basement conversion and it was all done to a totally finished state including new kitchen, bathroom and fire doors for just under £40,000 by a local builder!

    Premier Icon joebristol
    Full Member

    To add, we took the cheapest quote at what we paid. The next cheapest quote was £95k without the bathroom / flooring materials and another was £120k! I pointed out on that one that I wasn’t looking to build a whole new house….

    Oh, and get full drawings done before getting quotes. Otherwise what does the builder work on – they don’t know the amount of insulation required / how the roof is meant to work / foundation requirements etc.

    Premier Icon aberdeenlune
    Free Member

    I had an extension done at the back of our house about 5 years ago for the open plan kitchen diner and utility room. Decided to double it up with a bedroom and en-suite on top. Cost was £120k plus VAT. Size was around 5 x 5 m for each floor so about £2.4K per square meter but that was fully fitted out with a kitchen and granite worktops for utility too and my wife’s choice of £1000 toilet for the en suite 😂.

    The killer with a big extension is the VAT. That’s why for me a new build appeals once you get into the more expensive extension territory.

    Premier Icon oldmanmtb2
    Free Member

    Just get Grand Designs to film it, tell them you have a budget of £20k and it will come in at £28k……

    Premier Icon stumpy01
    Full Member

    Regarding the comments about getting drawings done first, how much is that likely to cost.

    I have an idea about extending the back of our house, but there is a chimney breast bang in the middle of the rear wall.
    I’d also want to move the separate kitchen and dining room at the front of the house to the new extension as a large kitchen/diner, then knock through the current kitchen and dining room and turn it into the living room.

    I imagine this would be quite a lot of work as the utilities will all need moving, on top of all the usual extension work and we’d obviously need a new kitchen as well.

    I just have no idea how much that is likely to cost, which will obviously affect whether its something that we could consider or not. If it’s gonna be too expensive it seems pointless even bothering with drawings…

    Premier Icon weeksy
    Full Member

    Wow… seeing these figures it’s often seeming more worthwhile to actually move house than it would be to extend !

    Premier Icon trail_rat
    Free Member

    Regarding the comments about getting drawings done first, how much is that likely to cost.

    Going by comments on here …. Nothing because DIY – 7k. For the sort of extensions being talked about here. Generally a % of the build value

    I was on the upper end of this. But I got initial visit to concept to planning consent in 4 weeks and then the build its self had no issues raise them selves as between the architect and the engineer they had covered them.

    The stories on here were good about some basic issues cropping up during the build or planning getting rejected for being in the wrong format. – there was a common theme -not using someone who knows how to play the game.

    Mate of mine has been bouncing drawings back and forth with planning for a simple internal wall removal . His drawings look great but he is missing the details they want to see . Wind load is his issue needs vertical steels as well as horizontals

    builder I know built to the plans on the “not an architect’s” drawings.

    Building control rejected it for being too close to boundary. The arbitration sided with the builder.

    So I do believe you get what you pay for. But it also pays to be able to understand what your looking at rather than just accepting they are good at their job…… That goes for who ever does the drawings. – even if just checking the basics.

    And finally. Ask around. If anyone in your street already has what you want ask who did the drawings and what issues were encountered -ground conditions are generally assumed based on the area on most small -medium extensions without site visit and this leads to alot of frustration when they are not as assumed and costs go up.

    Premier Icon intheborders
    Free Member

    Wow… seeing these figures it’s often seeming more worthwhile to actually move house than it would be to extend !

    Especially as was pointed out you pay VAT on extensions but not new houses.

    FWIW we renovated a cattle shed as an annex for my Mum a few years ago. Ended up at 100m2.

    We already owned the land and the building plus had services (just needed additional power). No change from £160k. Would’ve been far cheaper to knock it down and build as new.

    Premier Icon TheBrick
    Free Member

    Especially as was pointed out you pay VAT on extensions but not new houses.

    I think this rule is so poor. Encourage throw away rather than update and reuse.

    Premier Icon intheborders
    Free Member

    Especially as was pointed out you pay VAT on extensions but not new houses.

    And s/h ones too.

    Premier Icon joebristol
    Full Member

    If you can find a small builder you maybe able to avoid some VAT. I.e if it’s the one job they do over a year or one of 2 jobs that enable them to stay under the VAT threshold. You purchase the high value materials directly etc so it’s not part of their turnover. I assume this is legit…..but you never know with builders.

    If you use any meaningful kind of firm then this approach won’t work as they’ll have to be vat registered.

    On the drawings cost I want to say ours were a few thousand – but there are more coats to consider than just the drawings. Because we have a lot of glass we had to have the opinion of a company that calculate the thermal efficiency etc and tell you if it passes a certain standard. Our architect handled all the planning applications so they charged for that + the charge levied by the planning department. Loads of things cropped up that were a few hundred pounds here and there – plus they advised using private building control rather than the local authority ones.

    Planning got approved straight away / first time and the building control all went smoothly. The build took longer than hoped but that seems standard with almost all work we’ve had done ever.

    Other things to think about:

    If you need gas / electricity meters this is a ball ache. I had to get both moved – there are 2 separate companies that you’ll have to engage (not your utility providers) for each. They’ll connect the new boxes to the mains – but you have to arrange your own electrician / gas engineer to then connect from the new box location to the house. Don’t underestimate the time and cost for this – probably £3k or so I think from memory.

    Then also does your existing boiler have enough power for the size of the extension + existing house? We found ours was actually under sized for the house as it was (previous owners had gone cheap) – so there was another £3-4K for a new one. Plus we found the existing pipes to supply this were within the cavity of the external walls and weren’t big enough diameter to run a powerful enough boiler. That was a real arsehole to rectify.

    If the new extension needs water / sewerage then that’ll heap on further costs too.

    It’s a minefield – but what we’ve ended up with is ace.

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