Hire a geotech firm with test boring capability. Have them drill a few test borings and provide a report with allowable bearing capacity. With your construction info or examination of foundations you should have sufficient info to answer the question.

...and see if it will support a 9 storey building, too. You don't want to re-run the tests in 20 years.

Dik

What oldestguy suggests is getting someone to dig up some of the foundations and investigate exactly how they were constructed. 20 years is a long time and there's probably no one around who remembers or knows how they were built. If you have original design drawings that is great - but verifying the construction after 20 years might be a prudent thing to do.

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If there ae no old plans finding the size of old footings is no big deal. A back-hoe can do a few pits in a short time. In addition hand driven "soundings" with threaded well rod works well. Spaced probes will find the edges of footings. If you can't pull them by hand, a simple jack can be made for the job. Another method I have used is farm jacks about 40" long (from farm stores) and a length of chain. So you ask how do you attach the chain to the rod? A short story here.

A friend of mine in ham radio needed to pull an 8 ft long ground rod. So I exposed about 10 inches of the rod and wrapped a chain around with maybe 4 turns and attached the other part to the jack. the ham friend then says "That will never work". I said "how much will you bet, any amount?". He didn't answer. Sure enuff out came the rod.

What if the building is on piles.......

A 9 storey concrete building may or may not be on piles. Pile integrity testing would tell you the depth but getting access to a pile may not be possible.

I would start with the assumption that the 9 storey information is a fairy tale, and that no living person knows the true history of this abandoned building.

There are no building permit records? I agree on the fairy tale concerns. If it really was intended to have been a 9 story building, there should be records of it as such. I'll bet money that the site may have been zoned for a 9 story, but if a 2 story was built I would strongly suspect that it was designed as a 2 story.

At best, the 9 story was converted to a 2 story, because why? The original project ran into financial problems? That wouldn't make me comfortable that it was built to plan. Oh wait, we know it wasn't to plan. Run, don't walk away from this client.

Sometimes, when owners learn the cost of structural investigations, they miraculously find as-built documentation (or actual spend some time looking)

They started out by feeding you two stories, now nine?

Which story are you going to believe?

Do not be mister nice guy here without verification on this one, either by testing or original paperwork.

Just walk away.

Mike McCann, PE, SE (WA)