Are H1 B Non Immigrant visa holders allowed to purchase homes in the US, assuming they have the necessary income and a good down payment (20-30%)? Will the immigration status affect approval for the loan?
Your immigration status is irrelevant. I've purchased multiple homes in the US while being on multiple statuses, and no-one cared. The only place where it does matter is the bank, if you're taking a mortgage. Banks generally want a significant horizon of commitment, and that includes your commitment to being in the US. Some banks won't allow mortgages for H1b statues, others will. You'll need to shop around if you need a loan.
I am not a(n immigration) lawyer. You should consult one, if only for the conveyancing.
I was a non-immigrant O1 visa holder, and I purchased a house within a month of taking up residence in the USA.
Apart from requiring a SSN, the money, and suitable representation (a lawyer), there were no issues with me being a non-immigrant.
I am not sure whether this also applies to the H1B category.