Cost of single v double storey extension

[LovingTheSunshine]

Hi
I'm in Herts and we are contemplating an extension to create a new kitchen/diner/family room across the back of the house and a bedroom/ensuite above. I believe that it doesn't cost double to do a 2 storey extension - is this right? Is a lot of money in the foundations? Thanks

Hi OP - I'm a structural engineer.
You're absolutely right in that the cost of two storeys is less than double the cost of just one; you're already paying for foundations and a roof so it's just an extra floor and walls. The foundations may need to be marginally bigger but not twice as much.

There can be big costs in the ground or it can be very straightforward. It all depends on what you're building on - it could be good ground or mush! How old is your house? Does the ground ever get waterlogged, and are there any trees nearby?

A couple of things to watch out.for when you come to do it. It is highly likely that your extension will have separate foundations to the main house rather than tying them together. This means that, over time, the new part will settle slightly (we're talking a cm or so). You therefore need a movement joint where the two buildings meet. Some times this is omitted and that's when cracks appear. DH and I looked at a house once which had had 'subsidence'. It hadn't. They'd put too shallow foundations in and fixed the extension to the main house without any movement. Extension settled and started to pull the back wall off! Cue an insurance claim and subsidence on the record.

Not getting to scare you, just its a bloody simple thing to avoid and so really frustrating when you see how many problems it can cause when Mr builder is long gone.

getting = trying

Am bumping into your thread as in exactly the same position with same extension ideas- no idea if we can afford it so anyone with a rough guide of costs would be incredibly helpful in helping my confused brain!!

Hi lastnight
Many thanks for your reply, most helpful. Our house was built in the 1930s & no the ground doesn't get waterlogged but we do have a couple of trees in the back garden, the nearest one about 40 feet away from the current conservatory. Interesting to learn about the movement joist, will definitely remember that advice. We are contemplating a double storey extension or just a single storey & instead spilt a bedroom upstairs to make two small doubles. The bedroom is 19'4" x 12' 10" (narrowing to 9'7" for half of of the room). Lots to think about & cost up!! Thanks for your help, much appreciated. :)

Trees sound a reasonable distance away - they tend to be more of an issue if building onto clay. Gravel is best - any pits near you as that's normally a good sign you won't need complicated foundations. If house is 30s then existing foundations could be variable so almost certain the new ones will be separate.
One option would be to size the foundations and ground floor as if there is going to be a second floor but build it in two.stages. You will pay a little more overall but it allows you to phase the work over a few years.

Good luck!

We are in the same position and also in Herts. We really need more downstairs space and I can't envisage how an extension would look upstairs. I think I may need to ask neighbours to have a look at the work they have done. We have a cherry tree only about 15 ft from the back of the house. Would this cause a problem?

We are in Herts and have just started the process of getting quotes in (based on a detailed spec). Two storey side extension and single storey rear extension. I think we are adding 66m2. Plus quite a lot of refurb work in the existing house. Typical 1930s semi. First one in is £150,000 plus VAT plus kitchen.

Our two-storey extension (48m2) is costing 89,500 + VAT. It's fairly straightforward as we're not adding bathrooms and kitchen. However, we are adding lots of other work such as extensive internal refurbishment/replastering and work on the roof. We're adding wet underfloor heating to the extension.

It was a fixed price quote to include all the finishings such as decorating, lighting and adding a block-paved drive.

The only unaccounted for extra that we've come across is the bloody boundary wall, which is a whole sage in itself.

Foundations and roof would have been exactly the same if the extension had been single story.

ours came in - 6 years ago - at £1500 per sqm finished

What rooms are you hoping to create/add mewkins, ixos & hittingabrickwall? We have enough room at the moment (4 bed semi with double storey side extension plus conservatory) but the kitchen is relatively small for the size of the house. We could knock through to the dining room but I think moving it all to the back of the hiuse to create a kitchen/diner/family room would be great...but then the dining room would less natural light as it's only source at the moment is through French door to the conservatory at the back. Upstairs we have 3 big doubles & a small 8'8" x 8' room which seems u fair for the 3rd child to have that when the siblings have big doubles. Reading it all back on paper it sounds a bit daft when we have what we need but then again if we are going to stay here for years we may as we'll be happy. Hey? :)

Those of you wondering how to extend...all planning applications for a number of years can now be accessed online. If you log onto your locaol council planning portal then a little bit of detective work based on houses you know are similar can give you the original and proposed plans (as well as any rejected submissions). Worth having a look for ideas!

Party walls can be a bit of a nightmare I'm afraid, especially if you don't have amenable neighbours!

£1500 is pretty good but prices have been going up recently as the construction market has just exploded - workers can command a good fee and there's a bit of a brick shortage! If you need bricks, worth getting a few approved by the planners and making sure there is a good supply.

LovingTheSunshine we are creating a family room downstairs and two bedrooms upstairs. To access the two bedroom we have had to sacrifice one bedroom, which will become an open plan office.

We currently have a small kitche next to a decent sized dining room. Eventually we will knock the two together and create an open-plan kitchen diner. However, that would have left us with a prettty small living room and no other living area or office space. A work area is a must as DH works from home two days a week and is currently squashed into a corner in the already congested dining room. DS has too many fitted cupboards in his room so has no space for a desk and is getting more and more homework.

The kitchen-diner project will have to wait a couple of years (or more) now after the garden wall disaster.

The house will go from being a 3-bed detached to a 4-bed.

It's an 'unusual' plot, but if you'd like to see the planning application (plus all the arguments that went on with the highways department) then I can send you a link to the council planning department's website and our PP application reference if you send me a PM.

Agh I don't know...currently a 3 bed semi with open plan downstairs (apart from hallway which is enclosed). Options are to open up the back of the house to create large kitchen diner with lots of light, a sofa and storage for toys etc.... we would then block off the living room at the front to create a cosy space.

Or. We could just add another room to the back of the house - a playroom on its own with a sofa etc. This would then be useful as a sort of den when the children (currently small) are older and have friends over.

We are in a fortunate position where we are being given some money with the stipulation of uaing it on the house. The money is enough for a decent extension but not enough to allow us to upsize as in the time we have lived here the property market has gone crazy and houses we could afford are few and far between. Plus I like this house and garden and it's near a great school and good secondary schools.

We are sort of a bit stuck knowing what to do for the best.

Lovingthesunshine overall we are adding a bedroom, a bathroom, a big study downstairs, a utility, downstairs loo. The current tiny kitchen will be knocked into the dining room and then another 3 m added to the back of this space running the whole width of the house resulting in a big kitchen diner with separate seating area.

mewkins it is good that you can ask to look at what your neighbour has your ideas sound good and I like the idea of blocking off the living room at the front...easier to keep a room tidy, hey?! And a playroom is always a good option too...wouldn't be without ours. We have toyed with the idea of whether to move in a couple of years but the way the property market has shot up I doubt we'd be able to afford much more than what we currently have here. Scary!

Hittingabrickwall I hope you can sort the boundary wall without too much hassle, it sounds stressful :( Would live to have a noses thanks...will shed you a PM. We are contemplating splitting a bedroom to create two bedrooms...one would be a nice square shape whereas the other would be an L shaped room because of where the doors would need to be. Tossing options around in my head to see whether this would be realistic & downloaded skeptchup but haven't found it user friendly so have given up. Think buying some good old graph paper tomorrow is a good idea!!

Was £150k + vat what you were anticipating ixos? I have no idea really how much building works costs...just that they costs a LOT. My friend is knocking down their conservatory & having a single storey extension to extend her kitchen into a kitchen/diner/family room & her quote is roughly £50k but am unsure if that includes vat. I have a headache already. LOL

Lastnight I'm in east Herts & don't think the planning department show the actual plans online unfortunately. I may be mistaken though but have never come across any....will have to check....

I've sent you a link to our planning application.

We had to create an L-shaped bedroom due to having to get around the old exterior wall and chimney. The room is about 8ft square with another 4ft by 3ft bit at the end. Walking around it, it's a usable 'guest bedroom' that will fit a double bed plus other bits. It's not enormous, but it's ok. As a child's bedroom, it would be fine.

Our extension came in at just over 1500 per sq m + VAT, but that included lighting, decorating, some flooring, external work such as patio/block paving and was fixed price based on detailed spec drawn up by architect. We have a project manager (the architect) who has been excellent.

He is working hard to help sort out the boundary wall issues!

Lovingthesunshine- I'm also in east herts! Have you been happy with builders you have used? That will be another headache for me as the guys we used for our bathroom did my head in in the end!

We are just in the planning stage mewkins :)

mewkins we have a builder friend if you need a recommendation? Haven't used him ourselves yet but others have and are happy.

We are in the process (planning application) of having a two storey garage conversion and kitchen extension done. A friends builder husband has said it's not the extension that will cost the money but the fixtures and fittings. We are already sourcing our new bathroom and kitchen by making use of the sales. He also didn't laugh hysterically when we said we had a budget of £75000, (in North Yorkshire) I might add that he did say that if builders don't want to do your work they will quote themselves out of the market, so don't be surprised if the quotes you get are wildly different!

We are doing a one storey extension for about £60k. Expensive but South East and lots of ground work and on side of hill.

Architect said single storey would be roughly £60k then the kitchen on top. Double storey (1/2 at the top) would be roughly £100k.