(MONEY Magazine) – The heated-up housing market may affect not only the size of your down payment but also another big sum you'll owe when you buy a home--the escrow account lenders make you fund to cover property taxes and certain insurance premiums for a year or more. Figuring how much to set aside is getting trickier.

Three years ago, the U.S. Department of Housing and Urban Development (HUD) limited how much mortgage lenders could force homeowners to tie up in escrow. The motive was to lighten the burden on home buyers, but it's had an unfortunate side effect: "payment shock," when the assessed value of your property--and hence your property tax--turns out to be much higher than what you put away in escrow.

How can that happen? Your property's value can rise quickly if you've bought in a real estate market that suddenly heats up. And if you've moved into a newly built home, your property may last have been assessed while it was vacant land; a house will obviously boost the value soon.

In late January, HUD fine-tuned its escrow calculation guidelines, recommending (but not requiring) that lenders collect escrow funds in monthly installments, not one annual payment. And HUD gave borrowers the option to overpay their escrow.

David Ginsberg, president of Baltimore mortgage consulting company Loantech (800-888-6781), advises paying your escrow monthly, even if that means a fee of up to 0.5% of your escrow balance. "Keep tabs on your monthly escrow analysis," he adds. If you have doubts about whether the account is big enough, check with your lender.

--Vanessa Richardson