Please critique my extension estimate

[FelTay]

Hello! I'm trying to budget for a 4x4m extension (filling out a square) to create an open plan kitchen-diner in the south west.
Please let me know what you think and if I've missed anything. All help is appreciated - thanks!

Architect’s drawing £1500
Structural engineer £500
Building work (inc plumbing, electrics and plastering £40k (based on £2k per sqm plus VAT)
Roof lantern £2800
2m Patio doors £1500
Building regs £600
Kitchen £14000 (£6k on units, £4k on appliances, £4k on worktops)
Flooring £1000
Lighting £500
Underfloor heating £6000
New boiler £5000
Build over Drain cert £400
New patio: £2500
Total so far:£76kish (sob)

I'd use a higher value for the builder figure, mine was more like 2.5k per new sqm
Boiler looks a bit over, patio a bit under so prob will balance out. Maybe up the flooring?
First job - get something the builder can estimate against and get some figures in. You won;t get firmer quotes until BR drawings are available though

I think your kitchen estimate is under what you will finally spend. I don’t know how big your patio is but our new one cost way loads more 15 years ago. Basically, you are probably looking at £80000+. It always costs more than you expect and you haven’t factored in tea and coffee for the builders. Also, are they going to use your loo or hire a Porterloo for the front garden?

I would say it looks OK, but would add :

Flooring - unless you're going for lino, £1k is too low, presumably you'll need to re-floor the space you're adding to so that its all uniform?

Building works - so the plumbing / electrics etc we were quoted for (at approx £2k/sqm) were things like white plastic switches & sockets / standard radiators / white pvc windows / standard doors & door furniture. If you want to upgrade any of those (eg we installed lovely column radiators, 1930s oak panelled doors, brushed chrome switches etc) then that will be on top. Maybe an extra £2-3k depending on the numbers involved?

Decoration (unless you're doing it all yourself?)

Fitting - you've included costs for lighting / flooring / kitchen etc - who is fitting all of those, or are your costs inclusive of fitting? If they are, then I think they're on the low wide.

I also think £1.5k for 2m patio doors is cheap.

You've also not budgeted for the furniture / fittings. Its very easy to get carried away once you have a lovely new space and if you haven't allowed for a new table & chairs / sofa etc or whatever you're going to have, it can be an issue.

Finally, are you living in the house whilst its done? Do you need to store furniture (we rented a little storage unit)? Don't forget to include those sorts of costs / an increase on your normal budget for food etc if you end up with more takeaways / easy food if you have to cope without a kitchen for a few weeks.

Where are you based as this will make a difference
It's not a bad starting budget for mid level spec.
Is the architect just drawing up something simple or is there a design element to it
(If it's just the construction drawings as you have the design already then it's probably about right)
Is it coming under permitted development or will it require planning permission (or are you unsure) - if it does there will be planning fees and costs for the planning drawings
If it's just simple construction drawings an architectural technician may be a more reasonable cost within your budget than an architect but check their qualifications (I'm an architect)
Will it need a party wall agreement - how agreeable are your neighbours as it might be worth budgeting for a party wall surveyor (hopefully not needed but they have a right to ask for one and for you to pay for it)
Some of these costs will depend very much on the level of spec and size ie: - I have an 8m x 4m patio outside my studio which cost about £2k all in as it was done as part of the building works so the excavation was done when they had the digger for the foundations. I chose the paving carefully for the budget as it's easy to get very expensive very fast.
However my kitchen was £30k (solid oak hand built)
The boiler should be less than that if done as part of the development unless you need new pipework as well.

Sorry reread and see you are in the SW so costs will be fairly high

I think the flooring is doable at that price if you choose carefully as it will be
4x4 - 16m2 say £30/m2 (loads of choice for porcelain tiles in that range or less- we've got a gorgeous one for £15) =480 double it for labour costs and add a little bit for grout and adhesive = £1k

It will be a new concrete floor so should be fairly level and as you will be installing underfloor heating there will be matting/ screeding included in that cost

Structural engineer costs will be higher. They are prob close to £100 an hour for a qualified structural engineer and then there will be the cost for drawings etc. Prob overall closer to a couple of grand.

Do you have a contingency, we had to spend £3k proving there were no bats in our house before we could start. Probably not relevant as single story but just an example of hidden costs. I think your build estimate at £2k m2 is a little low to be honest but you will get a nice kitchen for your budget. Have you got budget for accomodation or storage. The dust is unreal and although we lived in most the time there were bits even we couldn't hack. Stuff will get damaged if it's not out the way. What about budgeting for driveway, garden repair. There will be a bit that gets trashed. Patio quote seems low I always estimate £300 m2. And I don't see any reference to steels?

@Hardchoices depends on what the SE is needed for
If it's just the calcs for a steel where the extension opens through to the house then it's probably fine
If it's for something more complex or difficult foundations etc then it may be more

If you run the job yourselves, you'll need a porta loo and a few skips.

This is all incredibly helpful, thank you! Looking through these comments has made me realise that sadly our estimations are more of a low estimation than a middle!
I think we'll be going back to the drawing board to see if we can remodel our existing floorplan better instead... (Probably a new post for others' opinion soon!)

Add in contingency of 10%, and put your costs up to £2.5k per m2 and I think you'll probably be there. (And yes, assume you will spend the contingency so you will need to have it in 'cash', not just as a theoretical credit card.....)

Skips? Portaloo? Where will you live / cook during the build (i.e. any additional costs associated with this?)

I think it’s about right. We did a 3x3 kitchen diner extension in 2016 and all in it cost £42,000 but we didn’t have underfloor heating but did replace boiler and put in a huge range cooker and we moved an electricity meter which cost £1.9k alone. My husband worked as a labourer every Friday to bring costs down.

This is was £42,000 got us.

I think it is a healthy budget but you will get wildly varying quotes.
£16k for a kitchen.....if you go to DIY kitchens you could prob cut that. We got a kitchen and large utility with quartz for that.

Boiler is high...we have been quoted £2.5 k for a high spec large Vaillant with a couple less. That includes a new filter and replacement motorised valves.

UFH is high. We had full wet UFH inc screed for a 45m space for £4K

The rest looks about right but you might get cheaper

Only advice would be could you not have a traditional roof with Velux.....flat roofs all leak and depending on position for sun that may be an issue without blinds

Sorry finished too soon

We have a lean to type build. The new build was 35m to create a new space of total 60m. Included extensive and complex structural work and drilling down into existing foundations, creating a utility and large 4m sliding doors a d outside patio and paths. Total was £75k and included the kitchen which as above was high spec.

I think your estimate is fine. We're in the SW and had an extension very slightly bigger 2 years ago and ours came in at less than that. We had a pitched roof with 4 velux which I believe is more expensive than a flat roof with lantern. I think you've underestimated the flooring if you need to do more than just the new extension area although I think you've over calculated on the underfloor heating costs (electic or water?). Costs will also depend on the materials used, are you going for a wooden frame internally or breezeblocks. Concrete or suspended timber floor? There are so many variables it's best to have a builder quote you really.