A property tax proposal by Mayor Marion Barry contains a hidden increase averaging $183 for each of an estimated 43,000 rented homes in the city, D.C. auditor Matthew S. Watson has informed the City Council.

The same tax proposal by the mayor would bring significant tax reductions for many of the city's most valuable commercial properties, Deputy Auditor Carl H. Bergman said in an interview.

Topping the list of commercial properties getting reductions is the Watergate complex in Foggy Bottom, which Bergman estimated would have its tax bill cut by $134,615.

The tax changes would be triggered if the council approves a reclassification of properties proposed by the mayor, Watson said in a formal report to the council's Finance and Revenue Committee.

In a letter attached to Watson' report, the mayor defended his proposal as a way of taxing profits of rental landlords, many of whom live outside the city.

By his proposal, the mayor said, the city would be "limiting the lower [residential] tax rate benefits to owner-occupants only, as intended last year." He said the council should be sure, if it rejects his proposal, that landlords would "pass the benefits along to renters and not pocket the tax relief" for themselves.

The mayor's proposal would modify a system begun last year, when the city began taxing residential properties at a lower rate than apartments and commercial properties.

Because of a tight timetable, the council put rental properties-both single-family homes and apartment buildings with up to five units-in the residential category. For the tax bills to be sent out next fall, the mayor would shift them to the commercial category.

If the council agrees to the reclassification, the mayor said he would recommend cutting the residential tax rate from $1.54 to $1.36 and the commercial tax rate from $1.83 to $1.65 for each $100 of assessed value.

This would produce an average tax bill of $756 on 62,000 owner-occupied homes, up $53; an average bill of $813 on 43,000 rental units, up $186, and an average bill of $2,925 on 35,000 properties now classified as commercial, up $31.

Under a formula set by city law to reflect inflation, the tax yield from each category of real estate in the coming tax year cannot rise more than 7.03 percent.

Because of this. Watson's report showed that adding the $2.1 billion of assessed values on rental properties to the existing commercial category would pull down tax collections from office buildings, stores and apartments.

Bergman cited the Watergate, the city's second most valuable private property, as an example of how this would work. Although its assessment is the same as last year, it could expect its $1.37 million tax bill to drop by $134,614. Without the shift of classifications, its tax would rise, depending upon the rate ultimately adopted by the council.

Following are the 30 highest tax valuations for commercial and apartment properties in the District of Columbia proposed for the 1980 tax year as compared with 1979 (figures in millions of dollars):(TABLE) Building(COLUMN)1979(COLUMN)1980 L'Enfant Plaza(COLUMN)$69.1**(COLUMN)78.0 Watergate(COLUMN)74.8(COLUMN)74.8 Columbia Plaza(COLUMN)45.0(COLUMN)45.0 Nassif(COLUMN)41.4(COLUMN)44.3 Van Ness Center(COLUMN)42.3**(COLUMN)44.2 Doctors Hospital(COLUMN)33.5*(COLUMN)38.9 Washington Hilton(COLUMN)25.4(COLUMN)37.8 Washington Post(COLUMN)32.1(COLUMN)35.1 1800 M St. NW(COLUMN)32.5(COLUMN)34.4 Hyatt Regency Hotel(COLUMN)27.7**(COLUMN)30.1 400 N. Capitol Plaza(COLUMN)23.2(COLUMN)28.1 1801 K St. NW(COLUMN)24.5(COLUMN)26.5 Mazza Gallerie(COLUMN)22.9**(COLUMN)24.9 Sheraton Park(COLUMN)19.5**(COLUMN)24.0 SW Town Center(COLUMN)21.5(COLUMN)23.2 Statler Hilton(COLUMN)17.4(COLUMN)21.5 500 12th St. NE(COLUMN)18.7**(COLUMN)21.0 1900 Pa. Ave. NW(COLUMN)19.2(COLUMN)20.5 Universal Bldg.(COLUMN)20.0(COLUMN)20.0 Woodward & Lothrop(COLUMN)15.5**(COLUMN)17.8 2020 K St. NW(COLUMN)16.0(COLUMN)17.2 The Towers(COLUMN)16.8(COLUMN)17.2 Shoreham Americana(COLUMN)11.8(COLUMN)16.2 601 D St. NW(COLUMN)15.8(COLUMN)16.1 1666 K St. NW(COLUMN)15.0(COLUMN)15.9 Harbour Square(COLUMN)13.4(COLUMN)14.5 2000 L St. NW(COLUMN)13.2(COLUMN)14.4 1333 New Hampshire Ave.(COLUMN)13.9*.(COLUMN)14.1 2100 2nd St. SW(COLUMN)13.6(COLUMN)13.6 1730 Pa. Ave. NW(COLUMN)12.9(COLUMN)12.9(END TABLE)(FOOTNOTE)

* Unfinished Property

** Appealed and Reduced by the Board of Equalization (END FOOT)