Determining S-Corp Shareholder Basis

[timtax]

Hello!

I have an S-Corp client that I took over for starting with the 2017 tax year.  The company has been incorporated since 2009 and the books were in horrendous shape when I took them over.  Since the IRS introduced Form 7203, I've wanted to see if I can properly determine shareholder cost basis.  I'm currently asking the client to provide all prior K-1s starting with 2009.  If they are unable to provide K-1s from that long ago, what should I do to ensure the Form 7203 has correct information reported on it?

Thank you!

[dkh]

Wouldn't you want the tax returns not just the K1s?  If the client can't provide - what about requesting copies form prior preparer? 

[IRonMaN]

How many shareholders are involved?