Garage conversion...how do you know if the foundations are deep enough

[Longheath8]

Had an offer accepted on a house that we went ahead with as we thought it would be pretty each to convert half a double garage into a study.

However, having spoken to a local builder this morning, I'm having a wobble. He said they would need to check if the foundations were deep enough. If not, they would need to make them deep enough. This would obviously alter the price hugely. Is checking the foundations something that can only be done when you're in the house? How do they check?

You dig a small hole in the ground at the side of the wall to see how deep/ wide the concrete is , it’s the only way to know for sure .

?? Are you building above or just inside ?

If you’re building inside you wouldn’t need to check...

Yes just inside. So converting half of the existing garage. Adding a window and replacing the current double garage door to a single one. I have to admit, I thought foundations were only important if you were building above a garage but now I'm not sure

Potentially a couple of reasons why you need some information about the foundations. The front wall on a garage is basically a large opening for a garage door, rather than a solid wall. This is much lighter so sometimes there isn't actually a full foundation and just a thickening in the floor slab. When you put your new cavity wall here you need to know it is sitting on something suitable. Second reason is that the walls on a garage are usually just a single skin of brick and not a cavity wall. Again a cavity wall is heavier and they need to check that the foundation/edge of the slab are sufficient for you to install an additional skin of block work on to. A single skin isn't really suitable for a habitable room and wouldn't meet building regs in terms of insulation etc. There may be other reasons but those are the main issues that come to mind.

That makes a lot of sense actually. I will phone round some builders tomorrow and ask some more questions. Feel a bit silly assuming it would be quite simple and not too expensive. If rhe foundations aren't good enough, does anyone know if they have to rebuild the whole garage or juat add a deeper foundation on the part where the window would be?

I’ve seen lots done and never seen anybody need to alter foundations

You can use timber frame with cladding or thermolite blocks , both are lightweight and will provide the insulation you need , the garage floor will be strong enough ( cars are heavy )

Had two sets of builders and surveyors round. Neither mentioned foundations, FYI.

Speak to another builder. We did this and building regs didn’t even mention foundations, let alone our builder. Sounds like your guy doesn’t really know what he’s talking about. If it can hold a car it can hold a desk 🤷🏼‍♀️

I converted mine and only at ground level but local building regs made me check the foundations!