For District of Columbia homeowners who want $110 deducted from their real estate tax bills next fall, there is a form that must be filled out and mailed back soon.

But two members of the D.C. City Council voiced alarm last week that many taxpayers will overlook the form, and even if they don't, the forms could leave some residents in a muddle about when the forms are due.

The form that must be filled out is an "affidavit of ownership." It was attached to the back of the notice of property assessment for fiscal 1979 that was mailed last week to 147,000 owners of real estate. Only owner-occupied homes qualify for the tax saving; rented properties do not.

At a meeting of the City Council's Finance and Revenue Committee, Council Members Marion Barry Jr. (D-At large) and Polly Shackleton (D-Ward 3) criticized the D.C. Finance and Revenue Department's handling of the forms.

In a separate action, D.C. Auditor Matthew S. Watson contended that the forms and an accompanying brochure contain conflicting information that may confuse taxpayers.

Among other things, both Barry and Watson pointed out, the brochure says taxpayers have 30 days to return the affidavit while the forms themselves say taxpayers have 60 days. Since the material was mailed in undated envelopes, they said, there is no way for a taxpayer to reckon either 30 or 60 days.

Leo J. Ehrig Jr., deputy director of the Finance and Revenue Department, said the deadline date for returning the affidavits is April 24, no matter what the brochure or forms say. He said the council changed the tax law between the time the brochure was prepared and the forms were mailed.

He and Donald R. Beach, the department's associate director for assessment administration, appeared before the committee, which is headed by Barry. Beach said the department will be liberal in dealing with those who miss the April 24 deadline.

Despite the council members' fears of taxpayer confusion, Beach said the department received 17,000 signed affidavits within three days after the forms were mailed out - some of them improperly claiming exemptions on rental property.

Shackleton said those who read the forms closely probably will mail them back. But she said she talked to several Georgetown constituents who did not look closely at the forms and did not realize the affidavit was attached to the back of the assessment notices.

The affidavits are required - for the first time this year - for a taxpayer to qualify for what the city calls a "homestead exemption" of $6,000 on the assessment of owner-occupied homes, including condominium units.

This means that the first $6,000 of the value of each owner-occupied home is deducted before the amount of taxes due is calculated. On a home assessed at $36,000, the owner will pay taxes only on $30,000. At the current tax rate of $1.83 for each $100 of value, each owner-occupant would save a few cents less than $110 in taxes.

Both Barry and Mayor Walter E. Washington have proposed a tax relief package that would increase the exemption to $9,000 and lower the tax rate.

Last year, for real estate tax bills that are currently due, the city granted a $6,000 exemption to all single-family homes, whether they were occupied by their owners or rented to someone else. The exemption was automatic, without the need to file any form.

This year, when the council extended the exemption to condominiums, it restricted homestead exemptions to owner-occupied units. Officials said there is no way, other than requiring the filling out of a form, for the city to determine if a unit is occupied by the owner and qualifies for the exemption.

Beach said the forms improperly claiming exemptions on rental properties were spotted because some property owners sent back several affidavits covering different properties. Under the exemption program, a taxpayer is permitted to file only one form, he pointed out.

Of the 147,000 parcels of property in the city, about 100,000 are single-family units that received an automatic exemption last year. Officials estimated that the owner-occupancy restriction will reduce the number of exemptions to about 77,000.

For information about the exemptions, officials said taxpayers may call the Finance and Revenue Department at 629-5918. Those who may have mislaid or have not received exemption forms may telephone 629-2119.

In addition to the homestead exemption program, the city has a tax credit of up to $110 available for many low-income homeowners and renters and for moderate-income homeowners who are over age 62 or who are blind or disabled. Such a credit can be claimed on the D.C. income tax form, due April 15. For information about this program, taxpayers may call 629-3324.