Footing under garage to extend

[Leigh8721]

Hi i'm about to complete on a house and the garage conversion has been completed but would like to build bedroom ensuite on top of this do we know if seller would know if I'm able to do this or how do I find out. I'm really hoping that I don't need to knock down and start again. Also if I'm lucky and the foundations are in place does anyone have any idea of basic build cost just the walls and roof.

Your footings should be at least 1m deep and 600mm wide to build anything but it's entirely dependent on the site. If you had any large trees nearby for example you could be required to go down a LOT more. You basically won't know until the builder does some investigation.

It’s reasonably unlikely the garage has been built to withstand the weight of a floor above.
You can try asking the previous owners, but I think you probably need a structural engineer to tell you.

Garages are rarely built on footings adequate for a 2nd storey. Underpinning will probably be needed but only know if you go digging!

Agree with others; our next door neighbours wanted to do this, got builders round who said they'd need to knock it down and start again. It had already been converted, they liked the space etc, so they had a loft conversion instead.

HI thanks for all responses very helpful. I had considered loft conversion but would really like to go on top of garage instead if possible.
Loofah01 you mentioned underpinning does anyone know the cost involved for this.
Also any ideas of cost to just get the shell built, i.e. walls roof and windows

Circa £2k a square metre as a basic starting point but all sorts can affect this

We looked into this recently re a house we were interested in.

In a nutshell, we found we couldn't get a definitive answer without buying it (as you'd need to get someone to dig to check current foundations).

I think if you are considering buying a house to extend over the garage you need to do so expecting to knock it all down and start again.

I'm not convinced underpinning would be a cost effective solution either, if the garage is built according to minimal regs for a garage. I am no expert though!

Thanks for all responses looks like I’m not going to be lucky as I had hoped for. Is a loft a cheaper and easier way?

I think even lofts vary a lot! Assuming you’ve enough height, then you need to see where trusses are. Also, is it a hipped roof? Do you need a dormer or is a velux enough? Pitch of the roof, height of building, fire regs etc.

We considered one on our house but it was going to be eye-wateringly expensive, particularly against the space we’d gain.

Thanks Saz wish I could get one positive message haha. We would need to put a dormer in

I’ve seen lots of people build over existing single stories , it’s definitely worth digging a hole first to see exactly what’s down there, you might get lucky.

The last single height extension we did we ended up going down 2.5 metres to hit solid ground, just filled it all in with concrete, you could now build the fking Taj Mahal on top of it. 🕌

We're in a similar situation although we'll definitely be building it. I'm hoping we can keep the main garage and just build on top. It's an attached double garage 🤞🤞🤞

If the seller had the garage built they should might have a copy of the plans and specifications. That would mean a structural engineer would easily know what is needed.

Thanks chumleymouse do you have any idea of coatings than I may roughly be looking at?
Paul I believe the garage was built the same time as house and then they have converted the garage

If it’s an attached garage already then I suspect you’ll find the foundations will be better than normal for a garage since they’ll have done them all the same depth. Are there other houses the same on your street that have been extended upwards?

The reality is though that if you need to extend then you’re taking a risk

There are only two of these houses on street the rest are mainly bungalows and different style houses than this one. I know we have taken the risk and if needs be will have to rebuild or do the loft