While interest rates mount and the federal bullet is bitten to combat inflation, the metropolitan Washington area shows nearly universal signs of continuing ebullience in construction and real estate investment this year.

Nonetheless, the three-year boom in sales of new and existing dwellings may cool in the first half of this year. Also, rental apartment construction will continue sluggish in the private market unless new federal lending subsidies are provided. And a continuing escalation of lendingrates and any lessening of lendable funds could also affect some planned major projects.

More than 30 large projects are planned for downtown and for some of the torridly expanding exurban areas such as Tysons Corner and Reston. I-66 and Rte. 50 (where the vast Fair Oaks shopping mall has been started), Woodbridge (where another major mall is planned), the I-270 corridor in Montgomery County and still available prime Beltway sites in Prince George's County.

Despite fears of realty reverberations in the real estate market from attempts to control inflation, most professionals agree with Coldwell Banker's James B.O'Brien that "this area is truly unique in terms of economic growth because each year Washington increases its stature as the place to be."

For the most part, O'Brien (a transplanted Southern Californian), leading realty developer Oliver T. Carr (the new president of the Metropolitan Board of Trade) and other realty activists remain confident that the inner city will continue to renew itself substantially while viable suburban centers expand.

Two cases in point:

A research-devlopment firm in the Westpari-Westgate section of Tysons is planning a move into 260,000-square-foot quarters. Two years ago it had opened with 30,000 feet of office space. That's a dramatic example of the expansion that is pushing downtown and nearby office building occupancy to record levels -- in a era of record high space rates. No longer is $14 a square foot annual rent shocking inprime downtown nor is $10 to $12 a square foot for top locations outside center city.

With all the financing problems for completing the Metro system and getting the midtown convention center under roof, the activation of the long-gestating Pennsylavnia Avenue renewal plan indicates that the rebirth of the old downtown (east of 15th Street NW and south of Massachusetts Avenue) will see some ground-breaking in 1979 and then move ahead significantly in the first half of the next decade.

Already at least one major trade association is highly interested in relocation there. And escalating prices of tired but stratiegic property sites (one was sold for less than $65 a square foot only two years ago) indicate that assemblies are being made for privately financed new structures that could stem, but not totally forestall, the recent westward expansion of the new downtown toward Georgetown, where major waterfront mixed-use rehabilitation is under way. And there will be more office buildings, hotels and intown shopping malls in balanced multi-use complexes.

More than one realty pro sees the rebirth of the jungle that now dominates 14th Street north of New York Avenue as being triggered by the plans for the adjacent Garfinckel block and other areas above and below the Metro station at I Street. In fact, every Metro station area seems likely to benefit.

There's the planned reconstitution of the southwest corner of Connecticut and L into a shopping-hotel-office center. And you can see new construction around the Orange Line terminus at Metro East-New Carrollton in Prince George's.

A renewal of Silver Spring is also likely now that Metro has enlivened that middle-aged downtown. Friendship Heights and White Flint already havestrong identities. And one can anticipate even more development in the Rosslyn, Skyline, Pentagon and Crystal City areas of nearby Virginia. Even the Metro-served Catholci University area has attracted new intown residential plans.

But what about the flurry of new office building construction already visible in the new downtown centered north of Farragut Square? It will continue as remaining renewable sites are assembled and replanned. McPherson Square has already assumed new importance with one big air rights building (developed by John Akridge) being built on its north flandk. And another jog northward, the southeast sector of Thomas Circle will have several new office buildings within a few years.

The economic headiness of downtown and fairly close-in office leasing is demonstrated by 1978 tenant signings that have eclipsed 3 million square feet while occupancy of existing spaces is realistically regarded as being between one and 3 percent. New spaces are taken in voracious quantities as the juggernaut of attorneys, associations and accountants continues through midtown.

In fact, fears of rising rates and confidence-spurred plans for future expansions have prompted many of the big space takers (law firms, in particular) to lease more space than they need to have it for tomorrow. Professionals say hardly a week passes without another out-of-town law firm inquiring about setting up shop here. And most of them follow through to get that "Washington window." Some small towns have distinctive "barrister rows," but ours seems more and more to be a City of Counselors.

Coldwell Banker downtown leasing specialist Hal Bowles pointed out that the annual demand for new downtown office space was unrequited by belowaverage construction in 1977 and 1978. He added that 1979 finished space (estimated at 1.4 million square feet) will leave a carried-over demand in 1980, when new space is expected to total 1.9 million square feet.

With the 1979 demand likely to exceed 2 million additional feet, there are plausible assumptions that new space demands will increase in 1980 and into 1981. That's because 83 percent of the new buildings to be complted this year are already leased; 32 percent of the space in buildings scheduled to be finished next year has been committed.

Meantime, costs of new construction and operation are expected to rise. Thus, tenants will be signing future contracts in the range of $15 to $16 a swuare foot by 1980. If the optimistic leasing specialist views, borne out by Braedon, Julien J. Studley and others, tend to appear to be an extension of Metropolitan Board of Trade publicity, they can be tempered with a review of leasing history. It is highly cyclical.

In periods of short supply, new plans are conceived and thenhatched. Avilable mortgage money germinates new buildings. So far, the money is still there, as the long-term ratesedge toward 9 3/4 percent and terms increase beyond 30 years to make the annual "constant" (repayment timetable) more acceptable. It must be recognized that the totality of all planned new projects in downtown cannot ever be tightly catalogued in an upbeat market. Thus, unless some deterrents emerge, overbuilding could be visible in late 1981 or 1982.

Although this is obviously a government-dominated town, little of the new office space, and certainly none of the new retail and commercial spaces being created, are dependent on federal leasing. For the most part, the General Services Administration has been building its own spaces in recent years. But a cutback in federal spending plans and any expansions of federal employment here, although now seen unlikely, could bring Uncle Sam into the space market. This becomes particularly true if there is oversaturation and rates become more competitive as the pressures to fill up completed buildings become acute for developers.

To summarize, a recent survey story for the Washington Board of Realtors showed the 1979 forecast to be "mostly sunny." But American University's Maury Seldin, a newly named rsearch fellow of the Urban Land Institute, was listed as the minority voice. He said: "Tightness of money will choke off much construction. It will be more difficult for builders, developers and home buyers to get financing. There will be a recession (for both commercial and residential construction) by September 1979."

As usual, this area's construction year will likely be better than else-where. Totally, it will probably be closer to "peachy-keen" than to "the pits." Most of the objective realty observers with good credentials refuse to doff their traditionally upbeat forecasts. But several insited that they be put under the umbrella marked "cautiously optimistic."