A new development is catching home buyers off guard as the spring sales season gets under way: Bidding wars are back. So just like an Auction, we are seeing more than one set of buyers and as many as 7-9 sets of buyers bidding on some homes. From California to Florida, many buyers are increasingly competing for the same house. Unlike the bidding wars that typified the go-go years and largely reflected surging sales, today's are a result of supply shortages. [BIDWARS] Peter Earl McCollough for The Wall Street Journal Debbie and Bill Wetherell received multiple offers for their home. Read More Is it time to buy your first home? "It's a little surprising because we thought bidding wars were done with," said Andy Aley, who is looking to buy his first home in Seattle's Beacon Hill neighborhood. The 31-year-old attorney was outbid this year when he offered up to $23,000 above the $357,000 listing price and agreed to waive inspections and other closing conditions. Competitive bidding in the current environment isn't producing huge price increases or leaving sellers with hefty profits, as occurred during the housing boom. Still, the bidding wars caused by tight inventory provide the latest evidence that housing demand is starting to pick up after a six-year-long slump. An index that measures the number of contracts signed to purchase previously owned homes rose in March to its highest level in nearly two years, up 12.8% from a year ago and 4.1% from February, the National Association of Realtors reported on Thursday. "We very much believe we've hit bottom," said Ivy Zelman, chief executive of a research firm, who was among the first to warn of a downturn seven years ago. Earlier this week, she raised her home-price forecast for the year, calling for a 1% annual gain, up from a 1% decline. View Interactive The Wall Street Journal's quarterly survey found that the inventory of homes listed for sale declined sharply in all 28 markets tracked. Real-estate agents consider a market balanced when there is a six-month supply of homes for sale. At the height of the housing crisis, in 2008, there was an 11.1-months' supply. In March, there was a 6.3-months' supply. Inventory levels in many markets were at the lowest level in years. At the current pace of sales, it would take just 1.5 months to sell all the homes listed in Sacramento, Calif., and 2.4 months to sell all the homes listed in Phoenix. San Francisco and Washington, D.C., each have 3.4 months of supply, while Miami has 4.1 months of supply. Other markets have plenty of homes. Chicago, for example, has 9.4 months of supply, while New York's Long Island has 16.1 months of supply. Even in those markets, the number of houses for sale is edging down. Increased competition is frustrating buyers and their agents. "We're writing a record number of offers, but we're not seeing a record number of closings and that's because it's so competitive," said Glenn Kelman, chief executive of real-estate brokerage Redfin Corp. in Seattle with offices in 14 states. Nearly 83% of offers that Redfin agents have made on behalf of clients in the San Francisco Bay area this year and 71% in Southern California have had competing bids. Redfin represented a buyer that made the winning bid on a Gaithersburg, Md., home earlier this month after agreeing to adopt the dog of the seller, who was relocating and looking to find a new home for "Buddy," a white toy poodle. Inventories are declining for a number of reasons. Some sellers, unwilling to accept prices that are still down from their peak by one-third, are taking their homes off the market in anticipation of higher prices down the road. Meanwhile, investors have been outmaneuvering consumers for the best properties, often making cash offers that are quickly accepted by sellers. In addition, some economists say that inventory levels are being held artificially low because Fannie Mae, Freddie Mac and the nation's biggest banks have been slow to list for sale hundreds of thousands of foreclosed homes they currently own. The lenders slowed down foreclosure sales and repossessions after record-keeping abuses surfaced 18 months ago. Banks and other mortgage investors owned nearly 450,000 foreclosed properties at the end of March, and another two million mortgages were in some stage of foreclosure. Inventories could rise, putting more pressure on prices, if the banks and other lenders step up their efforts to sell their properties. Real-estate agents say they aren't concerned. "There's an enormous appetite for foreclosures. Release the inventory. It will sell," said Richard Smith, chief executive of Realogy Corp., which owns the Coldwell Banker and Century 21 real-estate brands. [BIDWARS] The declining inventory of older homes is spurring sales of new homes. New home sales are up 16% so far this year, compared with a year ago, while inventories of new homes fell in March to their lowest level since record keeping began in 1963. Meritage Homes Corp., a builder based in Scottsdale, Ariz., reported Thursday a 36% increase in orders for the quarter ending in March versus the previous-year period. Even though bidding wars are pushing prices higher, many homes are still selling for prices far lower than a few years ago. Increased demand is "entirely affordability driven, which tells me there will be strong resistance to price increases" by buyers, says Jeffrey Otteau, president of Otteau Valuation Group, an East Brunswick, N.J., appraisal firm. Rents are rising at a time when mortgage rates have fallen to very low levels. The result is that the monthly mortgage payment on a median-priced home is lower than any time since the 1990s. Freddie Mac reported on Thursday that mortgage rates fell to 3.88% for the average 30-year fixed rate mortgage, near its lowest recorded level. Rates are "so low that we can afford a house that was out of our price range before," said Aarthi Srinivasan, who is looking with her husband for a home around Palo Alto, Calif., one of the country's hottest real-estate markets. Ms. Srinivasan says she fears that prices are being bid up too quickly. She says she had her "aha moment" earlier this year while touring a 50-year-old house that needed extensive remodeling. The home, listed at $1.1 million, received nearly 10 offers and eventually went under contract for more than $1.3 million to a buyer who hadn't even viewed the property. "There are only so many buyers who are going to be in such a hurry, so we're hoping it'll top off soon," she says. On Monday, they offered to pay more than the $1.2 million list price for a four-bedroom, bank-owned foreclosure. They haven't found out if they made the top bid. On the other side of those transactions are sellers like Debbie and Bill Wetherell, who had 17 offers in four days for their four-bedroom home in Danville, Calif. "I was floored. It was so fast, it was surreal," says Ms. Wetherell. The home sold on Wednesday for $796,000, more than $50,000 above the asking price. Still, the sale is for nearly $180,000 less than what they paid for the house in 2005. Ms. Wetherell's husband has commuted to Reno, Nev., for five years and they have decided to relocate. Housing markets face other headwinds. More than 11 million homeowners owe more than their home is worth. It is a big reason that the "trade-up" market has been stalled. These homeowners can't sell their current homes, let alone come up with the down payment for their next home. Mortgage-lending standards remain tough. Real-estate agents say an unusually high share of deals are falling apart because homes won't appraise at the price that buyers have agreed to pay sellers. Still, borrowers with stable jobs are looking to make deals. Kelly Pajela-Fu and her husband offered to pay the asking price of $600,000 for a four-bedroom home in Marblehead, Mass., within a day of the property hitting the market. "We just knew this house would go quickly," says Ms. Pajela-Fu, a 31-year-old doctor who had lost out on an earlier offer. Their strategy to avoid a bidding war paid off: The sellers accepted their offer before having an open house. Write to Nick Timiraos at nick.timiraos@wsj.com A version of this article appeared April 27, 2012, on page A1 in some U.S. editions of The Wall Street Journal, with the headline: Stunned Home Buyers Find The Bidding Wars Are Back.