You can double your money in four to seven years by buying land subdivided into large lots within 30 miles of the beltway.

That can be provided:

The economic patterns of the last 15 years here are a reasonable guide for the future.

You're prepared to plunk down a larger percentage of the purchase price than you typically would in buying a house.

You're ready for high carrying costs over a sustained period, such as interest rates of 10 1/4 per cent and up, compressed 10-year amortization schedules or balloon payments - with no off-setting income or attractive tax breaks.

You've done your homework on the land you're buying. This means checking out its selling price in relation to going market values and being status in the local master plan. You should also be aware of the land's current and prospective zoning, as well as all convenants that may inhibit its use. Make certain of the key physical characteristics that are fundamental to its value, such as soil composition, percolation and the like.

You're willing and able to take the inevitable risk that your homework was wrong or the Washington area's economic expansion was shakier that anyone understood, in which case you could be stuck with an overpriced piece of turf.

With all these precautions, appraisers and consultants here agree that purchasing a one- to five-acre, subdivided lot in the Washington orbit can be a very sound, competitive investment.

It probably won't return the high-leverage, after-tax yields available from investments in prime residential rental property, nor the high monthly income available from a portfolio of second mortgages. Dollar for dollar, it probably would never produce as big a speculative splash as a parcel of undeveloped, unsubdivided land 125 miles from Washington bought for song.

But for small investors in the affluent real estate market here - especially those who might want to build year-round residences on their land some day - buying such acreage can be an intelligent move.

A survey of land prices in this region suggests why.

Subdivided lots in the high-cost Great Falls area beyond McLean that sold for $750 an acre in the early 1940s and $1,500 an acre in 1958 now go for $20,000 to $25,000 an acre, typically in parcels no smaller than two to five acres.

These lots are not hooked up to gas lines or to sewage systems, despite their promixity to developed areas. They have roads, access to electrical lines and plenty of trees, but not much else other than location. Parcels throughout the area go for a minimum of $37,500 to $45,000 for two acres and from $50,000 to $100,000 or more for five acres.

In 1973, the same two-acre lots were selling for $14,000 to $15,000, and the five-acre parcels for $28,000 to $50,000.

Similar lots across the Potamac in Maryland have moved up as fast or faster, doubling in price, in some cases, in just a few years. Elsewhere in Northern Virginia, prices on small lots have gone up at a rate of between 20 and 35 percent a year. One secretary bought in Fairfax County's Shadow Walk subdivision in June 1974 for $27,000, for instance, and sold it 30 months later for $40,500.

Lots farther afield, like those in Maryland's Calvert and Anne Arundel counties, have also more than doubled in price in five years. Three-acre parcels in northern Calvert, carved out of farmland five years ago and sold for $9,500, are being re-sold now by the original purchasers for $20,000 to $25,000 each. Two-acre parcels in the rolling corn and tobacco country of Anne Arundel south of Annapolis that went for $14,000 to $15,000 four years ago are now being bought for $30,000 and up.

"The pace of the increase in subdivided land values outside the beltway (but within commuting distance of D.C.) has amazed even us appraisers," says John W. Fincahm, president of a Beltsville-based appraisal firm. "And we're not supposed to be surprised in our business."

"I see no reason why the increases won't remain high for at least the next several years," says Fincham. "All the market conditions and pressure are going to hold here - that is, assuming we don't have a nationwide depression or something cataclysmic like that."

Of particularly high potential in the coming five years, according to Fincham's analysis, will be acreage that offers "combined" residential and recreational possibilities for the emerging leisure-oriented American lifestyle.

By that, he means a lot which are prospective year-round home sites with a reasonable commuting time to D.C., but that are also located close to the family's preferred recreational area - the Chesapeake Bay, the Potomac or the mountains of Virginia orMaryland.

Next week's concluding segment: Financing land purchases.