Allstate Insurance Co., citing high theft losses and low ceilings on rates it can charge, is now refusing to write any new renters' policies in the District of Columbia unless the policyholder can guarantee someone will be home 24 hours a day.

District residents who already hold Allstate policies are not affected by the new requirement, which was adopted in November. But the company is turning down Allstate policyholders who move into the District and try to transfer their renters' coverage, as well as District residents applying for renters' coverage for the first time--unless they satisfy the 24-hour rule.

Allstate's action represents a departure from the way it typically operates in the rest of the country and. contrasts with the practices of other major insurers, such as State Farm, Aetna, Geico and Fireman's Insurance Co. Spokesmen for those companies all expressed surprise at the Allstate move and said they are continuing to insure renters in the District regardless of whether they are at home around the clock.

James Montgomery, acting superintendent of the D.C. Department of Insurance, has expressed concern about possibly unfair discrimination against renters who work or attend school and therefore cannot qualify for Allstate coverage. He said he will meet with Allstate representatives Wednesday to discuss the 24-hour rule, which makes exceptions for things such as vacations and nights out but prohibits coverage if the policyholder has regularly scheduled absences from the home during the day.

"Hopefully, I can work something out informally with the company that will be acceptable rather than take formal action," said Montgomery, who can impose a fine or revoke or suspend the license of a company found in violation of local insurance laws against discrimination.

Allstate said its rule is not discriminatory. "We are in full compliance with the law," said David Rowe, a spokesman for the company, which now insures an estimated 3,000 renters in the District. He said that the rule applies only to renters, not house or condominium owners, and that the company has no plans to expand the restriction at this time.

Rowe said that the company, one of the giants in the insurance industry, has lost money for two years on its D.C. renters' insurance, which covers loss or damage of personal possessions. "Our losses and expenses were such that we were spending $1.30 for every $1 of premium taken in," Rowe said.

Those losses, he said, reflected thefts from D.C. homes. Rowe said that national studies have indicated that the losses from property left unattended for long periods of time generally are greater than losses from property occupied 24 hours a day.

At one point Allstate filed a petition with the D.C. Insurance Department asking for a higher rate ceiling on renters' insurance, so that the company could adequately recover expenses and losses, Rowe said, but the request was denied. He said the only other alternative was for Allstate to limit its exposure to renters' theft losses by refusing to issue new policies to tenants who are not home all day.

Allstate and other insurers normally offer two different renters' policies at two different prices. Renters who have someone on the premises 24 hours a day typically are eligible for the company's standard, less-expensive policy. Those who are absent part of the day are offered the higher-risk policy, which may cost as much as 50 percent more than the standard coverage.

In Arlington, for example, the Allstate standard renters' one-year policy for contents worth $30,000 in a brick high-rise apartment sells for $106 while the Allstate Indemnity higher-risk policy for the same tenant would be $160.

But in the District, where the cap on renters' insurance for contents worth $30,000 is $107, Allstate traditionally has sold only the standard policy. Until November, when the rule changed, that policy was available to renters regardless of the number of hours they spent at home.

One 34-year-old woman who moved here recently from Memphis lost her Allstate policy because of the occupancy rule. She said she had been with Allstate for 10 years and had never filed a claim. But when she contacted the company about transferring her renters' policy to her new Chevy Chase apartment, she was turned down because of the 24-hour occupancy requirement.

The woman checked with several other companies and finally bought a tenant policy from one of them.

The other major insurers in the District, scratching their heads over Allstate's decision, say they intend to keep offering renters' insurance without the occupancy requirement.

Geico, for example, which has 4,252 renters' policies in force in the District, "will write almost any kind of policy as long as the property is in good shape and we don't have too much concentration in one area," according to company vice president August Alegi. As for Allstate's move, he said, "I really don't see their rationale."

Aetna, with 3,326 renters' and condominium contents' policies, does not have the 24-hour rule, said spokesman Bob Nolan.

State Farm, although it reports a near doubling of losses from tenant policyholder theft claims between 1979 and 1981 in D.C., says it has no such rule and no immediate plans to impose one.

"Cutting off new business is kind of an extreme step that we don't like to take unless we have to," said David Hurst, a company spokesman. "It would give our agents one less product to sell in the marketplace . . . and our loss experience isn't bad enough for us to do that."