Thousands of District residents and businesses who receive their mail in post office boxes soon may get an unwelcome letter from the U.S. Postal Service: a notice that the rent on their boxes will increase.

But for the first time in at least 15 years, the U.S. Postal Rate Commission is proposing that the size of the fee increase should depend on where the post office is located.

Basically, the higher the real estate value, the higher the cost.

Boxes in post offices across most of the nation would go up an average of 29 percent to between $17.50 and $130 for six months.

But in the downtown area, Foggy Bottom, Georgetown, Union Station and Dupont Circle, rates could rise by average of 31 percent, to between $19.50 and $140.

The Pentagon also would get the higher rate.

"The significance of this is that it is more market-orientated pricing," said Hugh McGonigle, a market specialist for the Postal Service.

McGonigle said post office boxes, like any real estate, are more expensive to provide in neighborhoods with higher real estate values. As a result, the Postal Service has no incentive to provide more boxes, although demand is growing.

The rate increase will help some, but the Postal Service still will lose money on its boxes in these neighborhoods, he said.

The decision will be handed down by the Postal Service Board of Governors Tuesday as part of a package of postal rate increases intended to raise revenue for the Postal Service. The Postal Rate Commission also is proposing to raise the cost of a first-class stamp to 29 cents, a 15 percent increase likely to become effective Feb. 3.

The higher charges for boxes in post offices would affect upscale neighborhoods in the Washington area and seven other metropolitan regions.

In these neighborhoods, the increase would amount to $9 a year for the smallest boxes and $80 a year for the largest.

Bethesda, McLean and sections of New York City, Boston, Philadelphia, Chicago, San Francisco, Los Angeles, Beverly Hills and Honolulu also would be in this classification.

In Manhattan, where residents can wait months for a post office box, the Postal Service is proposing to increase the six-month fee to $21.