Is building above a garage cheaper than a ground floor extension?

[EachpeachpearplumIspytomthumb]

I’m hoping yes!? We have a regular 3 bed semi which was already extended with a brick built garage to the side. We are thinking about building a bedroom on top of this. Assuming as the foundations etc are already built, would this be cheaper than building a similar footprint size rear ground floor extension (which we’ve had quotes for almost £100k for 😭).

Why do you think it would be cheaper?

Because the foundations and groundwork might be already done as stated in the OP??
I'd definitely expect it to be quicker to get the work completed OP but you might need deeper foundations for a second floor extension than for a simple garage. Worth quoting for both though!

The foundations might not have been engineered for a two storey building. You would need to get them checked.

I doubt that the foundations would be deep enough for a two storey build. Assuming the previous owner put up the garage. They would not have spent more than they needed to on their own project.

We had to demolish the garage, dig deeper and more extensive foundations, then build from scratch, so unlikely to be a cheaper option.

I was trying to get an idea of the OP's thought process!

Likely to be more. The foundations are unlikely to be good enough, and even if they might be no builder is likely to take the liability of building on top of someone elses work without making doubly sure, so probably under pinning to improve the foundations. You'll also likely have a more complex roof and junction/interface with the existing building depedning on the exact design. You'll also potentially need additional stairs or to move existingn depending on the layout of the first floor.

Also, what are you hoping to gain? A single storey ground floor suggests you need extra living/cooking/dining space, first floor suggest a bedroom/bathroom, so what do you want?

Ideally, both.😆
Ok I was hoping if the foundations were good enough it would be cheaper. But sounds like perhaps not. We just can’t afford 100k for a smallish extension (which we aren’t in desperate need of anyway) so was thinking about the next thing on our list which was another bedroom/re do the bathroom.

Would a loft conversion be cheaper? I think they're more like £40-50k (at least that's what my old boss got quoted last year in the SE).

Foundations may well be deep enough. They may have future proofed it to leave open the option of an upstairs extension in future. That’s what my parents once did.

I have it done two years ago and our foundation was deep enough. It was built on the same foundation pad as the main house so the structural engineer was very sure of it. Despite this, the cost was nearly £100k. As a PP said, we needed half the roof taken off and rebuilt. It's not a small job. The new second floor is on top of the garage and part of the kitchen. The builder added temporary support in our existing kitchen so they can walk on top of our existing joist without falling into our kitchen. The other builder who quoted the job wanted to demolish half our kitchen. That might have been cheaper to build the extension but will cost us more because we'll need to refit a new kitchen.

My parents thought this ended up being cheaper to pull garage down and start over.
Until you have done a test pit on the foundations you won't know the answer.

No it won't be cheaper. The garage will likely need underpinning or demolition and rebuilding.

Unlikely to be cheaper unfortunately! But it does depend. Your best bet would be to get an actual quote.