The Maryland Attorney General's office ruled yesterday that Prince George's County cannot legally use its tax-cutting TRIM amendment to establish a "tier tax," under which homeowners would pay lower tax rate than apartment and commercial property owners.

Any effort to set up such a tier tax structure would require a change in state law because the county does not now have the power to differentiate between commercial and residential property for taxing purposes, according to the opinion written by Assistant Attorney General Kaye Brooks.

An attorney general's opinion, however, does not carry the weight of a court opinion, Brooks said. It is advisory to state and local officials.

The TRIM amendment, passed in a referendum last November, prevents the county from collecting more taxes in future years than it did in 1979.

That would set the county's maximum yearly tax revenues at around $140 million.

Another provision of the TRIM amendment, gave the County Council authority -- through the county charter -- to make TRIM applicable only to residential property.

TRIM's promoters saw this as a means of easing the tax burden on homeowners whose property assessments have been rising at a fasterments and commercial properties, according to Del. David Bird (D-Prince George's) an organizer of the TRIM movement.

The constitutionality of the entire TRIM amendment still is uncertain, Brooks pointed out, because questions have been raised as to whether TRIM will interfere with the county's bonding contracts.

In order to sell bonds, the county must pledge that it has unlimited taxing power, which would serve as security for the bonds.

Brooks said the attorney general's office has not been asked to issue a ruling on the bonding question, but the County Council has asked the General Assembly for the authority to set up a special fund to serve as security for any new bonds.

It is also unclear whether the TRIM ceiling applies to the taxes the county collects to help fund the budgets of the bi-county Maryland National Capital Park and Planning Commission and the Washington Suburban Sanitary Commission. An attorney general's opinion on that matter is expected on Friday, Brooks said.

Bird said yesterday that he will seek further clarification on the attorney general's opinion relating to the tier tax structure.

A spolesman for County Executive Lawrence Hogan said the executive had not yet seen the opinion. The spokesman said Hogan opposes any form of a tier tax since it is unfair to cut residential property taxes and not commercial taxes at a time when the county is trying to attract new business.

County officials already have said that TRIM will not lower the property tax bills of most homeowners this year or for many years to come.

The real beneficiaries of TRIM, county officials have determined, will be owners of stores, factories, ware-houses and apartments, whose property taxes, will in most cases, drop every year.

Many homeowners will have bills that increases by as much as 7 percent, according to the county's chief tax assessor.

Bird said that homeowners' tax bills will rise, on the average, only 2 percent instead of 12 percent because of TRIM.

TRIM's other originator, William E. Goodman, said he opposes a tier tax since he believes that fiscal belt tightening in the government eventually will cut down on the need to increase taxes.

He noted that because of TRIM, the county's property tax rate will be cut this year from $3.31 to $3.05.

Assessments will continue to rise, however, according to the rate of inflation, and drive overall tax bills up.