Anyone built on top of existing single storey extension?

[JoanDarc]

We’re in the early stages of considering options for extending. Standard 3 bed detached early 1980’s Barratts house which has a single storey side lean extension, approx 20 yrs old.
Interested to hear if anyone has built on top of existing extension and experience of costs for doing this. Ideally we would build slightly out as well increasing current extension which is 2.6 m to 4m wide and then build up. Any experiences much appreciated.

Bump - also interested

Assuming like a garage extension with 2 stories as it’s not designed to be structural it would be a case of removing the current extension and starting the build from ground level up again?

It wasn’t a garage extension, it houses our kitchen, utility and downstairs toilet currently. It looks to be doable to add another floor if the foundations are strong enough. Think we need a structural engineer to confirm this by digging a trench. Interested for anyone who has experienced this and costs vs knocking down and starting from scratch.

Yes, but we had done the lower extension ourselves a couple of years before, so knew what we were working with. We chose to have a timber frame extension. My parents did the same over their garage, but during the time between having the downstairs build and the upstairs built, the insulation gap requirements had changed, so they had to have 10 cm insulation added to the inside of the room. Having timber frame gives you a much slimmer wall.

Do you have the plans for your extension? Ours showed that we had the appropriate foundations to add a story.

@Yellowiceytek thank you. Do you know how it impacted the cost? I’ve seen online that rough estimate say first floor is half the price of a ground floor extension but no idea what a calculation would be for first floor on top of existing? So allow £2k per sq m ground and £1k per sq m first floor.

Foundations and existing construction are key. Are there building control records or detailed plans held anywhere so you know what was built? Is it standard brick construction?

Building costs are around £2000 per m2. Not sure about demolition but it could be demolished pretty quickly and piled into a skip. If you don’t want the same footprint, I would start again.

We built above an existing detached garage but by the time you have taken the roof off and demolished what you need to, there’s not much saving. I’d start again and get what you want.

@AngieBolen we’ve only started to consider doing an extension so haven’t engaged an architect yet.
@PresentingPercy I’ll try and find out this info. Think I have some historical documents sent with the deeds.

I’ve found the plans of the old extension, it was actually built in 1987. Plans refer to foundations being 600 x 150 mm concrete strip. No idea if this means anything to anyone.

We have a single storey extension, that we would have liked to be taller. I'm fairly certain the slab is 1m deep, with insulation on top of that (2008 build).

15cm sounds like the sort of thing you would pop a conservatory on .

I think £1k per sq m for an extension even of it's just 1st floor is unrealistic.

If you need foundations and the room is to be finished to a good standard you need to allow £2000 per m2. There is also garden refurbishment too.

A slab won’t be 1m deep. The foundations might be. No need for floors to be that deep.

600 x 150 for foundations on a 2 story extension doesn't sound like something that's meet current building regs.

On our build I'm pretty sure our foundations were nearer to 30cm wide and at least 600mm deep

We live in a clay soil area though, which I believe affects the foundations you need.

I think that means 600mm wide but only 150 mm deep. That’s not very deep. See attached. I think you need to consult an engineer to see if this is suitable for a second storey. I really don’t know. Trench fill foundations would be normal here.

600x 150 is very shallow for foundations, the single story we just did needed to be 600 wide by 400 deep, but due to ground conditions ( old rubble and back filled loose clay when the house was built ) the foundations ended up being 600x2.5 meters deep.

You would need to dig a small hole next to the house to determine exactly how deep yours are.

15 x 600 doesn’t sound very much by modern standards, but it also depends on what you‘re putting on top - for example, a timber frame is much lighter than a brick wall. If you’re also increasing the footprint as well as adding a storey, it may be easier to just start from scratch.
To be honest, it would be better to ask an architect to help you assess feasibility and plan something appropriate as it’s hard to speculate without having seen the whole site and structure. The earlier they get involved the better, then you don’t end up going down paths that won’t work out in practice.

I think you get the plans, you dig your inspection hole and get the structural engineer to see, your architect draws up the build and then your builder quotes.

Alas we couldn't get planning to put a second story above our garage but that was our game plan.

Thanks everyone. I think the foundations aren’t strong enough ( it’s 60cm long by 15 cm wide buried 1.05m under so top of foundations is at least 45cm below ground according to plans) but speaking with a friend in construction today it sounds like they may be able to be reinforced/ added to IF that’s the route we want to go down. Unfortunately the side lean is only 2.6m wide so it wouldn’t be wide enough for what we require upstairs without increasing footprint anyway.
I’ve got a recommendation for an architect so will have discussions and think if we do want to pursue this, we need to get a structural engineer to assess as @didireallysaythat highlights. Thankfully we’ve got the foundations info without doing a dig so can have more informed initial discussion. I’ve a feeling this is the beginning of a very long complicated process 🙈

Before you incur the cost of a structural engineer, it's worth getting some pre-application from the council as to whether you'll be able to go double storey, and if so, how far out you can go.

I'd get planning first - a favourable pre-app doesn't guarantee you'll get it.

our house had a single storey extension on the back when we moved. We built on top but we knew from previous planning permission that they had intended to add a first floor at one point, and they had had the foundations done at the time to accomodate that.

Most definitely take down and rebuild if the footprint of the ground floor extension is to be increased.
We were in the same position as part of a bigger extension and renovation and we knocked down the existing ground floor extension.

Those foundation measurements don’t make sense. Where does the 1m measurement come from. The measurement should be width (600mm), depth (150mm) snd length (2500?) or whatever. If there is depth in a trench to 1m plus these are not built as your original plans suggest. Which was the absolute bare minimum concrete strip.

Get your plans done and get pp. Your architect should know a suitable engineer to investigate and design appropriate foundations.