Cost of an extension

I know they’ll be a lot of ifs, buts and unknowns in this question, but I’m going to ask anyway to help me work out what’s feasible for us.

The long and short is we are a family of four: me, wife and two girls (six and nine). We live in a four bedroom house with one bathroom currently. With home working taking effect and two girls rapidly growing up and taking more and more ‘bathroom time’ we are thinking of either moving or extending. Moving’s been taken off of the cards - too much disruption and value for money is not great. So extending it is.

The plan is to build an extension on the side of the house adding a second lounge, master suite, two studies, boot room, and other spaces. It’d be build on the side of our existing house and would only require knocking through two walkways upstairs and downstairs, so minimal work there. It’s be two story but using the loft as living space, and high gabled roof to match the existing house. In total I was thinking it’d be 5 metre by 10 meters. We’d also be knocking down a double garage.

The trouble is the cost. We live in the south east of England - a village called Leeds outside of Maidstone. So costs would be higher, which on websites tells us would roughly be £1,800 per sqm meaning about £90,000. We’ve had a friend whose done a smaller build for roughly the same sqm cost as I’m thinking but my boss was building a porch and over lockdown the cost went from £20,000 to £100,000. Now I don’t know the size, quality, or whether the builder was chancing their luck and not really wanting the job in giving a high quote, but made me think about the true cost of doing such an extension.

My question is if people have done extensions of late what is the true cost per sqm? I’m planning on getting going with firming down plans in November time and need to start working out finances so any help dearly appreciated. Also I am aware of lots of ifs and buts around this in terms of quality of build and fittings (about middle on the line we’re thinking), but with so much variability is builds and costs thinking about true cost is really hard.

We are planning for precisely this. Taking down the garage and building a garden house of 5*10 metres. We’ve just received the first offer and it starts with 60k Euros for the bare construction and then additional costs for floor heating, luxury glass windows, roof windows et cetera. When I all add it up, it comes indeed close to 100k Euros. This is a high quality, wood based construction.

100k is significant money, but there are many benefits. It makes your living more comfortable and given a family with children may mean that you don’t need to move to a bigger place (cost saving). I does also increase the value of your place and giving the high inflation, its better to get rid of money as soon as possible and convert it into a building. Thats a structural investment.

Any building work always costs more than the quote and you might need to move out for the work to be done. Another cost to be included.
You need to see other work from builders you have short listed.


We had an 8x6m single story brick, apex roofed extension on the back of our place last year for pretty much bang on £1500 per sqm. 3 different builders were 20k apart in quotes. It’s a simple room mind, with minimal work to the existing property but I’ve understood that reasonable costs are £1500 per sqm for single story and £2500 for double story extensions are a rough guide. We’re over in East Suffolk

1 Like

We are in a very similar situation. Family of four with 2 young girls rapidly outgrowing our 3 bed detached house in Nottingham.

Our extension is due to start in June. Upstairs we are just adding another bedroom and en-suite but most rooms are actually moving around. Downstairs another lounge, utility, WC and just more space generally. We are extending the back of the house by 5 metres (8 metres wide) and coming out on the side (all the way down the house) by 2 metres. It’s a lot of work with, ultimately just one room of the current layout not changing in some way. It’s terrifying!!!

I got quotes from 5 building firms starting from £100k (1 man company with the work to take roughly 12 months) to £180k (a team of builders with the work taking 4 months). We have been planning this for a while and the cheapest rough price prior to the pandemic was £75k.

I am going to do a fair bit of work myself (build kitchen, tiling, flooring, woodwork & trim) because I can and want to. That is helping us save a few £££.

It’s a massive commitment to live in a building site for half a year and take on the finances but we are outgrowing the space we have and love our location. We looked at moving but the numbers and reasons stated led us to extend.

Good luck!

We have slummed it with just one bathroom since buying our house in 1992. We also have a downstairs loo. Looking back it was a bit inconvenient at times but as the boys became teenagers they had far less stuff. All the big plastic toys go and they spend a lot of time in their rooms doing homework and other stuff kids do. Then at 18 it’s off to university and now we have the house to ourselves. If you are balking at the cost I wonder if it’s possible just to go into the roof with a bedroom and bathroom? People seem to get a bit carried away with how many toilets they think they need. You need to consider the value of other houses in the road and not extend beyond what’s sensible. Roads have their limits as to what people will pay to live on them. You never want the most expensive house on your road.


Daughters are different in bathroom time needs (or demands anyway) than sons HH.


My dad was one of 10 and he, his brothers and sisters, and parents lived in a three bedroom house with an earth bucket as the only toilet. That of course is extreme, but there is a balance to be struck. People have gone en suite crazy. Anyway, we digress.

That reminds me of a film I saw on TV years ago, about a young woman recently widowed unexpectedly, who had discovered that her husband had spent all their money and her only asset now was a football team that was at a training camp out in the backwoods, miles from anywhere. She went to visit them and when she had introduced herself said “where’s the bathroom?” Someone replied “Outside”. She said “Where outside?” The answer came “Anywhere outside.”

I am staggered by the cost per metre that are being suggested at the moment. The Guardian gave some figures over recent weekends that are just not achieved in any tender I am involved in at the moment and haven’t been for some time.

I realise that I live and work in the ‘South’ but I would be recommending that a cost per sqm should start at £2500 for a simple single storey extension. The money is spent on three key areas, the structure and electrics etc which you won’t really see and has to meet all the current building regulation standards, then the windows/sliding doors/bi-folding doors and roof lights etc which can be aluminium, wood or UPVC (which I would never recommend but are cheaper) and then the internal fittings. Kitchens for instance can cost £5k from Ikea and £40k from other dedicated supplier. I always recommend that my clients take out the kitchen fittings from the core costs and then decide how much they want to spend on a kitchen and the fittings.

You then need to remember that the government charges 20% VAT on all this work and that needs to be factored into the budget.

You can then consider whether you pay a little extra now to ‘over’ insulate to save you money going forward.

Talk to several builders and I would recommend that you get the design done with expert help, and that you have a full set of documents to get builders prices so that all the prices start at a level. But then I would say that!

Current difficulties with getting timber and glass for instance mean that prices are highly volatile at the moment. The war in Ukraine is yet to have a significant impact but will very shortly and for quite a while going forward. I have had a roof light supplier say that they cannot guarantee a supply price until a matter of weeks before supply!

We moved here in Jan 2018 with the intention of “renovating” (1970s bungalow in South Lakes).

The architect assured us that £150k would cover it. Building work started April 2018 & we eventually moved back in 2 days before Xmas that year, after spending 6 months in a caravan. No kitchen or bathrooms finished & all the furniture in storage. Memorable Xmas.

Over £250k later it was finished (mostly) so my advice is get a fixed quote (if you can) in writing. And never believe what the architect says. I’d also think about the important stuff first - some things can be done later others have to go in first (electrics/plumbing/heating/insulation etc).

Good Builders up here are just as difficult to find as down South - and just as expensive.
We are actually now considering extending! I need somewhere soundproof for the music & being a bungalow that’s difficult. Thinking back I come out in cold sweats :roll_eyes:

I wish you luck.

HH makes a very good point that I always explain to my clients before they embark on a project.

The first is that the money you spend on the extension etc adds the same (or more!) value to the property. This can be tempered by the fact that the added value to the property may not be actually added for a year at least and that assumes a continued increase in property value which may not happen everywhere to the same level.

The second is that you spend the money because it brings you joy and makes your life easier and happier. For instance expensive kitchens rarely add that value to the property even over many years. They do however, usually get your house sold more quickly. But they do bring you joy using them etc.

You need to understand where you are spending your money so your future regrets are limited.

1 Like

The advantage of what I’m planning is that building it in the side we only have to shut down the driveway and partition a section of the garden off because of our dog. We shouldn’t need to leave the house at all, which will save us some money - just cope with the noise.

Thanks. It all gets more complicated as I look into it!

Good point. Thanks!


1 Like

Thanks! I always presume costs are inclusive of VAT so I’ll bare that in mind!

We’ve had various bits of work done in recent years - kitchen, bathroom, plastering and other stuff. The builder we use is my brother in law’s brother in law, so there is a sort of family connection. He’s not cheap and it’s all done on the books, as we’d want of course. He organised everything, including the electrician and plumber, and the skip, but I bought the stuff on his advice, and using his trade accounts. So it was kitchen units at Howdens and granite direct from a granite supplier. He tipped us off that if we ordered from Howdens at the right time they may well offer a big reduction just before delivery if it meant the delivery moved to the earlier month. These tips were invaluable and we knew we could go on holiday for a fortnight and leave them to get on with it. The labour and building materials price was fixed up front. I can’t imagine we’d have done it without such a brilliant builder. We’ve had some in the past who have been total cowboys and they could be pretty scary at times.


I’d echo the comments of many here about escalating costs. A friend here in North East Hampshire has just had a pre-Covid quote of £75k rise to now being £115k. The builder is good but is quoting huge increases in costs and availability of materials.
A single story garage I had rebuilt during Covid came in at £1500/m2 and would be greater now.

On the other side of the argument, unless you need to move home for family / employment reasons, I continue to believe that an extension gives you more payback than moving. The new place will probably need work and it is easy to forget the happiness you get from enjoying the extension, irrespective of what it does to overall value. Just don’t use up all the garden!

1 Like

My recommendations, which you can pick & choose from as, as you’ve already worked out, until the builders’ quotes come in, you can only really guess at what the ‘market cost’ of getting the works done will be.

1- speak to neighbours who’ve had similar works done (if any?) and get their reco’s and the details of who they used and any issues(?) - you may be able to piggy-back on their professional team, design facets (common template?) and other experiences.

A search of the local council’s planning register for local properties which have had works done should reveal the professional parties engaged and also any planning conditions applied. Early engagement with the planning team for anything which will require FULL PP is highly recommended.

2- be specific about your wishes and needs with your professional team e.g. architect/designer - they should be able to cost the project as part of their services and also have good relationships with local builders. You could even try going to a local builder direct and asking them for an indicative cost, and to recommend an architect/designer.

3- consider whether you might want the builder to only do the shell (to watertight), with you managing trades to complete. This can cost far less but the risks around time delays, execution et al are obvious.

4- If you are concerned about attaching to existing structure, local ground conditions and doing new foundations etc, then get a building engineer engaged pronto (perhaps via architect/designer/builder), as you need solid costs for getting ‘out of the ground’.

5- beware architects/designers who want to do CAD for you et al, with all the design input, as they sometimes don’t balance-up their thinking to budget IME i.e. you need to engineer-in any costs constraints at the outset.

6- when the time comes, make sure the works are monitored (e.g. by your architect) and progress is duly certified, which can then act as triggers to release funds.

Avoid thoughts of seeking the cheapest builder. A strong professional team is what you need and, unfortunately, that can cost more but it’s like an insurance policy.

I’ve seen far too many people get stitched-up in these activities, including some very close friends.