Very basic question re calculating extension cost

[MrsGethinJones]

I have a really basic question that I hope that a wiser-than-me mumsnetter can help with please!

We can extend out to our side return. The width of the side return is 2 metres. The length of the extension would be 10 metres making the new added floor space 20 square metres.

The existing space is 3m by 10m, so the new extension will be 5m by 10m.

The online extension calculators is asking me to add the ‘floor area of ground floor extension’. Should i be adding 20 square metres as this is what is being added, or the total new floor area which is 50 square metres??

There is a huge difference in price!

Just the extra that you're adding on.

Yes 20sqm as a starting point.

Ok great - thank you!!

But bear in mind that if you need to redecorate the existing ground floor once you’ve extended, lay new flooring/ underfloor heating etc, it will add a lot to the cost.

Can I jump on and ask what calculator you are using please? I want to extend to the side too but need to have a rough idea of cost before I even approach any builders.

A good average cost (outside SE) would be something around £1000 per square metre for building work - foundations, wall, glazing, roof etc. Then add whatever you fancy for floor coverings say £50 - £100 /m2, £5k to £30k for a kitchen £2k up for a bathroom. £5k and up for a boiler upgrade etc. If you use a local VAT registered contractor add 50%.

We are in SE and have spent just over £2k a M2 (priced with new m2 only) that's us pretty much done including some of the external works and re configuring some of the existing house. Finish is not high spec just a family home.

Thanks!

Thanks all!

I used the calculator on the build team website for a ball park figure.

The new area will be 50 sqm and will 20 sqm will be new space. The space willl include a new bathroom, bedroom and kitchen diner, all at ‘average’ finish in London - Im fine with an Ikea kitchen. Do you think doable for
80k all in

Yes, but you’ll need some contingency.

I think £1k per sq m that a previous poster mentioned is way too low, unless you live in a very cheap part of the country and can benefit from mates rates / trade discounts. We did an extension 4 yrs ago & spent roughly £2k per sq m (plus VAT) just to a plastered finish (kitchen bathroom flooring decorating plus garden etc all on top of that).

OP - £80k sounds like a lot but if you maybe work on £50k for building costs with VAT, you'll find that the bathroom, new kitchen, flooring will quickly eat up that £30k.

I think £80k is at the very lowest end for a ground floor extension in London, and possibly not doable. You may find that you struggle to attract good builders as they would rather mobilise their team on larger projects.

www.houzz.co.uk/magazine/how-much-does-an-extension-cost-stsetivw-vs~134210773

I was costing up filling in a side return and the online calculators came in at £80-100k, depending on kitchen and plumbing. The added area is only part of it- steels, kitchen, utilities, new flooring, glazing and so on.

We did pretty much the same - I kept a close eye on the budget but the extras do add up. And don't forget about professional fees as well - architect, structural engineer, party wall surveyor etc, if needed. £100 - £110K seems about right, as a minimum. If you have the skills you may be able to cut some costs by project managing yourself, but it's not for the faint-hearted.

Also, bear in mind that other parts of the house will need redecorating after you've finished, not just the areas that have been touched.

Lots of people use 2k per new sqm as a guide, I've found that 2.3-2.7k is more realistic especially in London or SE. I doubt the builders will include the kitchen, maybe the bathroom but be very wary of this is it will likely be the basest of basic spec. Get extremely specific about what is included in any quote! If in doubt, take it out. My builders were supposed to send over a complete breakdown of what's included but all I ever got was the broadest interpretation which put me in a mindset of 'they're going cheap on the finish' so I removed the bathrooms, decorating and kitchen and internal doors to sort myself (they still fitted) and was more specific on expectation of the windows and external doors. All worked out more or less.

Totally agree with above. Don’t get builders to include bathroom or kitchen fittings. They get the kitchen and bathroom ready for fitting. You do need plans for the kitchen and bathroom though so drainage is in the right place and electrics etc.

I'm having an extension done now (SW London) and £2K per m2 is about right for the basic 'shell' including wiring and plastering - as part of a bigger job that includes a rewire and re-plaster of the whole house.

Everything else, fitting a new bathroom and kitchen etc is on top of that.

£80K sounds a bit tight. Especially as all the builders I know have plenty of work, so the good ones don't need to price it low.

And I agree with the PP about excluding kitchen & bathroom fittings - choose your own and then you can decide where you are happy with budget items, and where you want to spend a bit more.

I was told £1500 per square metre was a good starting point. If you want a high end finish it will be higher, if you do some of the work yourself and are careful less but it's a good ballpark

This is good advice everyone thank you - I think i need to increase the budget as I definately dont want the builder chosing my kitchen for me!