Condominium apartments are being resold in the District for staggering profits -- often averaging 3 to 4 percent for every month a unit was owned following construction or conversion, a new city survey shows.

George B. Altoft, senior residential assessor for the District, said data on all condominium units resold for the first time during the first quarter of this year shows that some of the profits have been "truly phenomenal."

The most dramatic sales price increases have shown up in new projects where purchasers reserved units prior to construction, or in buildings converted from rental to condominium ownership, where tenants bought units at steep discounts.

New units reserved in 1978 at Westbridge project at 2555 Pennsylvania Ave. NW -- before it was built -- have been resold by individual owners this year for profits of 46 to 73 percent. Some of the sellers had owned the units for as short a time as three to six months.

Units at the Westbridge that initially sold for $117,000 to $138,000 late last year have appreciated by 12 to 15 percent a month and have been resold for an average of $202,000 this year, Altoft found. The profits for speculators who put down deposits of around $5,000 in 1978 -- to reserve units that would be completed late in 1979 -- have amounted to $60,000 or more.

Apartment resales at converted condominium buildings were less spectacular, but still very high. Units at the Tatham House at 2711 Ordway St. NW appreciated by around 5 percent a month over an eight-month period. Apartments originally purchased for $57,000 or $58,000 last year have been resold in 1980 for an average of $81,670, Altoft reported.

At the West End condominium, 1320 21st St. NW, the only unit to change hands this year sold for $100,000: Thirteen months earlier, its first owner bought it for about $54,000.

Units in other projects surveyed by Altoft turned by wide variations in price increases, with the average monthly appreciation tending to drop the longer the unit was owned. The typical resale took place after 14 to 15 months.

But a discounted, tenant-purchased unit in a converted building, Altoft noted, can often be sold almost immediately for a sizable profit, in some cases 25 to 35 percent more a month later, although the rate of appreciation declines the longer the apartment is held.

For instance, a unit in Foxhall East, at 4200 Massachusetts Ave. NW, appreciated by 2 percent a month for the first owner after it was purchased in 1977 for $115,000. It was sold earlier this year for $250,000.

A sample of other sales price increases noted by Altoft and the number of months between initial purchase and resale:

Georgetown Overlook, 1318 22nd St. NW, 65 percent, 21 months.

Duddington Manor, 115-117 E St. SE, 60 percent, 15 months.

Carrollsburg Square, 300 M St. NW, 74 percent, 41 months.

Capitol Park II, 600 Third St. NW, 65 percent, 43 months.

Dupont East, 1545 18th St. NW, 14 percent, three months.

Bonwit Plaza, 2401 H St. NW, 57 percent, five months.

Corcoran Mews, 1746-56 Corcoran St. NW, 27 percent, 17 months.

The Essex, 4701 Connecticut Ave. NW, 33 1/2 percent, 19 months.

Archbold Glover, 4000 Beecher St. NW, 43 percent, nine months.

Woodley Park Towers, 2737 Devonshire Pl. NW, 17 percent, 15 months.

Resale prices for condominiums in the District may not increase at anything like these percentages during 1980 as a whole, Altoft cautioned, because of financing problems and consumer resistance to sky-high prices at some projects.

Moderate-priced conversion units where tenants are offered large discounts from market prices, however, appear to have the potential for big resale profits in 1980 and 1981.