It’s likely to be a structural engineer, rather than architect for the foundation.
Can you find out who this was from the builder? As well as the architect.
I think (would have to confirm with my architect husband) architects are required to keep plans around for 10 years. I know this was an issue for his former boss when he was closing up shop… had to find a place to store 10 years worth of the firms plans.
The other option – although more expensive. Hire a structural engineer to inspect/assess. Since you’ll need their expertise when you get the new plans, perhaps you can work out a deal.
Anecdotally, I know my husband has told friends that it’s rare that a 1 story home can add a second story, except over the garage, without foundation work. But YMMV.
