Mortgage rates aren't as low as they've been all year, but after four consecutive weekly decreases, they do offer a new opportunity to lock in mortgage costs.

"Rates are not as low as they were in March, but if anyone missed that cycle, they could refinance now or jump in [as a home buyer] and take advantage," said Warren Myer, CEO of Myers Internet Services, a San Jose­based publisher of the LoanApp.com and BestRate.com consumer mortgage websites.

The average fixed interest rate on 30-year conforming mortgages stood at 5.81 percent on Aug. 19, after four straight weeks of declining.

The Aug. 19 rate isn't as low as the March 19 average of 5.38 percent—this year's low point—but it is much lower than the 6.24 percent average during the same period last year and this year's high of 6.34 percent, according to Freddie Mac's Weekly Primary Mortgage Market Survey.

Fixed interest rates in the West, including California, were even better at an average 5.76 percent.

"After earthquakes, lenders think California is not the best place to do business, but right now because loan sizes are larger [than the rest of the nation], you can get better prices. There's not a huge difference, but lenders also see the West as much more competitive. People shop harder," said Myer, who also studies consumer borrowing habits nationwide.

In any event, given this year's ups and downs in the mortgage interest rate market, an interest rate lock is a good way to secure a mortgage interest rate today for a loan that might not close weeks or even months down the road when rates could trend up.

"In the next 12 months, anticipate rates, including the cost of housing, to continue to increase," said Andrea Caldwell, with Century 21­Alpha in San Jose. "This is a small window of opportunity that, over the long haul, probably will be slammed shut," she added.

A rate lock is a lender's guarantee that your mortgage carries a specific interest rate, points and other costs. The lock is also good for a specific period—if you fail to complete your home purchase or refinance before the clock runs out, and interest rates rise, you'll pay the higher rate.

If interest rates fall during the lock period, you may not be able to take advantage of them without paying additional rate lock costs—unless the rate lock contract allows you to have the lower rate.

Rate lock policies and costs vary widely. Some lenders automatically extend the lock only for a set period even if your loan doesn't close on time. Other lenders will automatically extend the lock until the loan closes, perhaps with one free extension. Still others will charge a small fee to extend the lock.

Locks should be long enough, initially, to allow for settlements, contingencies and other processing. The average is 30 to 45 days.

"If you cannot find a property in a week and if you lock in only for a week, what good is the lock?" asked Jack Tereza, broker/owner of Brooktree Realty in San Jose.

Rate lock costs are just as variable from lender to lender. Some charge up-front fees, others levy fees at settlement, and still others charge a fraction of a percentage point on the loan's rate. Cost can also vary based on the length of the lock period, any options you might choose and the mortgage program itself.

With so many variables, a written lock is mandatory. Oral agreements won't withstand legal scrutiny, should it come to that.

The written lock contract should lock in as many variables as possible—the interest rate, points, initial lock date, lock period, lock cost, post-lock options and other details.

Lock as soon as you see the desired rate—"on application"; otherwise, you'll have to determine if you can risk rates rising as you wait for them to fall further. Locking on application is key if you barely qualify at today's rates and an increase in rates would make buying unaffordable.

In today's hot sellers' market, Tereza says the rate lock—a lender's promise for a given interest rate and other loan costs—goes hand in hand with a preapproval—a lender's promise to loan you a given amount.

"My buyers have to be preapproved before I will present their offer. When you submit your offer if you aren't preapproved and there are two others who are preapproved, you are dead," Tereza said.

Real estate writer Broderick Perkins, executive editor of San Jose-based DeadlineNews.Com, writes regularly for this newspaper.