Hillsborough County doesn't have a single house on the Gulf of Mexico and only about 2,000 that directly front a bay.

Nonetheless, there are thousands of other single-family homes — modest bungalows as well as million-dollar mansions — that could be hit with soaring flood insurance rates under a new federal law that takes effect Tuesday.

Following the lead of its Pinellas counterpart, the Hillsborough County Property Appraiser's Office on Wednesday released a map showing approximately 21,800 older homes that until now have qualified for lower, subsidized flood insurance premiums.

"I think we all tend to associate flood areas with the coastline, but the vast majority are not out on the coastline,'' said Pat Alesandrini, director of valuation for the property appraiser.

Only about 2,000 of the homes are considered "coastal'' — defined as those areas subject to flooding from waves on Tampa Bay, Hillsborough Bay or Old Tampa Bay. The rest are on or near lakes, rivers, even retention ponds.

"They're absolutely everywhere,'' Alesandrini said, noting that thousands sit inland in areas like Riverview, FishHawk Ranch, Plant City, Brandon, Seffner, Carrollwood and Lutz.

"They just kind of sprawl all over,'' he said.

All of them could see their discounted rates phased out as a result of the Biggert-Waters Act, passed by Congress in 2012 in an effort to return the National Flood Insurance Program to solvency.

The Hillsborough map does not show condos or businesses, but they too could be charged much higher rates.

Real estate agents say the law takes effect at a particularly inopportune time.

"It's kind of disheartening just as our market is turning around and there's appreciation going on for the first time in five or six years,'' said Jeanne Wolfe, a Smith & Associates agent with listings in South Tampa and Davis Islands, both in flood zones.

Travis Bell, an agent with Keller Williams, notes that prospective buyers used to order home and termite inspections before thinking about insurance. But now, more and more are paying up to $200 for a flood elevation certificate, so they can get an insurance quote before proceeding.

"It's an added cost,'' he said, "but we want to make sure the insurance costs are not astronomical.''

Bell sweated it out as the contract on a non-waterfront home in South Tampa's Sunset Park nearly fell through when the buyers discovered the annual flood insurance premium would shoot up to $7,700, up from the current $1,900.

"They said, 'We can't do this deal, we're going to back out,' '' Bell recalled. "So we all got together and said, 'What if we closed in September and did a lease-back until Oct. 14?' '' with the current owners renting the property until then.

By closing before parts of the law take effect on Tuesday, the rate will increase no more than 25 percent this year instead of tripling all at once. But the new owners will have to pay the full rates when the policy renews next year.

Insurance agents say they are receiving more calls than usual from prospective home buyers.

"Everyone is just so confused, and there are so many what-if scenarios,'' said Charity Huerta, personal lines manager for Italiano Insurance Services in Tampa.

Not surprisingly, the homes facing the biggest rate increases are those with "negative'' elevations, meaning they are above sea level but below the flood level established by the federal government. But premiums vary widely because so many other factors go into setting rates, including the grade of surrounding property and even whether the air-conditioner unit is raised off the ground.

Among the older, non-waterfront homes for which Huerta has recently quoted rates:

• A 3,866-square-foot house on Watrous Avenue in South Tampa. The current owners pay $2,895 for flood insurance. The buyers, who are closing Oct. 30, will pay $4,885 — a 69 percent increase.

• A 1,980-square-foot home on San Carlos Street, also in South Tampa, that is closing this month. The premium currently is $2,841, and it will increase no more than 25 percent. Had the buyer waited until after Oct. 1 to close, it would have been $6,811 — a 140 percent increase.

• A 1,720-square-foot home on Geneva Place on Davis Islands. The owner doesn't have a mortgage, so he wasn't required to have flood insurance. He recently decided to buy it anyway for $5,200 a year. Had he purchased a flood policy two years ago when he bought the house, he could have renewed this year for $2,151.

While those three houses are low-lying, Daniel Orsello's home in the Citrus Park area of northwest Hillsborough is 38 feet above sea level and has never flooded since it was built in 1957, he says. Yet the 65-year-old retired trucker is close enough to a retention pond that his mortgage company requires him to have flood coverage, which jumped from $900 a few years ago to the current $1,175.

Orsello, who gets $580 a month in Social Security, wonders how he'll manage another increase when it comes time to renew in July.

"I would never have purchased this house had I known anything like this was going to happen,'' said Orsello, who has owned the home, valued at just over $100,000, since 1999.

Across the bay, the Pinellas property appraiser released its own map last week showing that as many as 33,114 single-family homes in the county — more than one in seven — could face soaring premiums as their subsidized rates are phased out. Contrary to a popular perception, most of the affected Pinellas properties are not gulf-front McMansions but more modest homes, some of them well inland.

Whether in Pinellas, Hillsborough or elsewhere in Florida, the new law is bad news, many fear.

"I think it's a concern for the banks if people can't afford to stay in their homes when insurance premiums go up so much,'' said Wolfe, the Tampa agent. "I cringe at the thought of seeing more foreclosures and short sales come on the market.''

Times researcher Connie Humburg contributed to this report. Susan Taylor Martin can be contacted at susan@tampabay.com.