What's location worth? Nearly $40 a square foot on a long-term lease to Perpetual Federal Savings and Loan Association, which is leasing 1,900 square feet on the ground level of the new 1101 Connecticut Ave. NW building being completed over the Metro station there. The Charles E. Smith firm is handling leasing in the attractive, red-brick structure for developer Gerald Miller. Tenants, among them an Arthur Adler store, will begin moving in this summer.

Downtown leasing specialists have been talking about the interest two CBS and ABC are showing in being principal tenants - or, possibly, owners - of new buildings likely to be developed east of Connecticut Avenue and north of Pennsylvania Avenue NW. That's another indication of the vibrancy of downtown real estate development and the likelihood of increasing new commercial redevelopment east of 15th Street within a few years. After all, the surge to the West has gone about as far as it can go.

In Arlington, mortgage banker Roy D. Hanson has moved out of the three-story brick building at 116 Wilson Blvd. that he has worked in for 23 years. 11 other buildings in that area had been cleared for the next year. But work on Metro had damaged the sole remaining structure enough to force evacuation. Hanson, who was an original tenant, has moved to the second floor of 2334 Wilson. He said Metro may fix up, resell or raze his old bailiwick after subway contruction is completed.

The 145 apartments in 4101 Cathedral Ave. NW, which is converting to condominium ownership, have been sold out, according to co-developer John Mason, whose partner is Stuart Bernstein. Mason said that 73 percent of the dwellings were sold to tenants during the eight-month sales period. The average price per square foot of living space in the 16-year-old improved building was $51. Mason denies a report that he is buying the Towers apartment building on Cathedral for a conversionl to condominium ownership.

"How to Buy a Home," a practical guide to finding and buying the right home, has been published by Follett Publishing Co., Chicago.The paper-back by Connie Fletcher is available for $5.95. This analysis of the housing market, prices, location, financing, interiors, exteriors, contracts, closing, condominiums, mobile houses is a solid primer for persons new to home ownership. Maybe the best advice is in a "final word" item: "Whether you're in the market for a Victorian manor, a suburban split-level, a condominium, a cooperative or a mobile home, your search for housing will always lead in unpredictable directions. The stantdards and practices involved in real estate transactions are forever changing. Exceptions to the rule are anything but exceptional."

A. M. Byers Co. recently sold a 21-acre parcel at Old Springhouse Road near the Dulles highway for $6 million. The purchaser was Westgate-Old Springhouse Road Associaties Limited Partnership.