The value of taxable real estate in the District of Columbia -- half the property in the city -- will rise 14-1/2 percent, to $14.2 billion, in the next fiscal year, city officials estimate. The increase will be smaller than the 15 percent rise in 1979-80 and the 17-1/2 percent jump in 1978-79.

According to estimates by the Department of Finance and Revenue, residential dwellings and commercial property -- offices, large apartment buildings -- show the biggest appreciation. Detached houses are registering a 21-1/2 percent increase, to $4.3 billion. Rental properties and similar investment structures are in the same growth category. Commercial buildings, appreciating by 21 percent, are now worth $6.6 million to the tax assessors. Multi-family housing, with two or five units, trails behind with a 5 percent increase, to $3.6 billion.

With adjustments such as homestead exemptions, assessment appeals and deduction of new construction, the actual tax base for fiscal 1981 is estimated to be $13.7 billion. In the current fiscal year, the District raised $189.4 million on a $12.3 billion tax base.

For fiscal 1981 the city is expecting revenues of $203.3 million, or 7.3 percent more than in fiscal 1980. To raise amount next year, the District government is projecting a tax rate for detached houses of $1.08 per $100 of assessed value, down from the current $1.22.

For example, if a house was assessed at $100,000 in 1980, the tax was $1,110. (The first $9,000 of value benefits from a homestead exemption.) If the house appreciated by 21 1/2 percent this year, the tax after the homestead exemption will be $1,215.

Owner-occupied rental buldings with fewer than five units will be taxed at this rate as well. Small and large apartment houses will be assessed a uniform $1.60 per $100, while other commerical property will get a reduction of four cents in their rate, to $1.79. (These projections are subject to later revision, but the Department of Finance and Revenue does not expect them to go much higher.)

In fiscal 1979 the Watergate was Washington's most expensive property for tax purposes, being assessed at $75 million. In fiscal 1980, L'Enfant Plaza, valued at $78 million, captured the top spot. Which of the two will have the distinction of being the most valuable taxable property in fiscal 1981 will be known next month when assessments are mailed out.