what should down payment be on a 900,000 purchase of commercial building?

Depends on the loan product, use, property, and borrower profile. It could be anywhere from 10% (SBA) to 40%. Have you talked to a lender yet? Only they can tell you what product will work in your situation and required down-payment.

Actually depends on the type of building to start with. Multi-family, strip mall, office bldg., Industrial warehouse? Then you have owner/user etc? It gets complex especially when you step outside the standard lines such as FNMA, SBA (Government supported entities) or even CRA, then there are different "conventional lenders" and whether you have experience. Best advice do independent research and compare. Good luck.

Much more info needed to advise you properly, but in general, there are no "conforming" set of guidelines for commercial loans.. each lender that lends on commercial properties can make up their own rules and guidelines.. One lender might want 70% loan to value, while another might go as high as 90%.. If you own a business, and the building will be used in your business, then SBA might be a good choice.. But you cannot use SBA for commercial real estate investing.. you have to have a business use (other than renting it out) in order to use their financing. Each property stands on its own. The lender will look at your business, and what role the new property will play.. manufacturing plant, storage warehouse, industrial, strip mall, office complex, gas station, convenience store, etc, etc, etc.. Will you be owner occupying the building or renting it out? Again, lots and lots of missing information.. the short answer is there is definitely options, but without knowing all the details, it's impossible to advise you.. I'm a preferred Lender with California and Arizona being my primary markets. If you or someone you know is looking for financing options, feel free to contact me or pass along my information. 480-287-5714 WilliamAcres.com NMLS# 226347 / RPM Mortgage NMLS 1541014 / AZMB0121893