The big, stone house on a hillside in this halmet northeast of Baltimore has walls more than a foot thick, central air conditioning and a heating system divided into four zones that may be operated independently. It is also equipped with a security alarm system, intercoms in all major rooms and a telephone that rings outside as well as inside.

But the home of Carol and Jim Trela is not a modern contractor's dream present to a son or daughter.

The Trela house is one of the oldest in the Free State, with an original portion - now a kitchen with fireplace - that dates back to the early 1700s.

Moreover, in these days of great interest in roots, the Trelas have found a house with its history already detailed in 250 years of leases, deeds, letters and wills culled from old government records.

The first lease they found, for example, is dated June 26, 1786, in which 200 acres were rented to William Baldwin for 10 years, at an annual rate of 18 Maryland pounds. The Baldwins lived here for more than a century thereafter, giving the community its name.

Carol Trela is an antique dealer, specializing in 18th and 19th Century American furniture. She and her husband, a professor of sociology at the University of Maryland's Baltimore campus, wanted a house that would compliment their taste in furniture and one big enough to include a showroom for the antiques they sell.

The Trelas, who moved in with their young son Eric last fall after living in a ranch-style house near Reisterstown, Md., are not quite sure how old their new home is. Records go back to the 1730s, when the house, named after an early owner, was known as Brookes Cross.

At various stages over 250 years, additions were built from large stones found on the property. The house has been out of ths Balwin family only since 1951, although the surrounding 200 acres of the original homestead has been reduced to 13.

The Trelas have uncovered evidence of a rich history and are finding out even more as they work on plans to restore a semi-modern dining room to an earlier decor.

They know, for instance that there was once a covered bridge on the property - when they settled on the house they had to sign an ancient covenant giving rights to a steam for travelers and cattle.

The red barn, once much larger, has three sides beautifully wainscoated inside. There is a smaller stone barn, a carriage house, stairs leading to an ice cellar and a 30-foot, solid stone well. What makes the house on Baldwin Mill Road truly unique, however, are the modern touches added some 15 years ago, by previous owners. Reinforced - but hidden - steel beams were installed throughout, for example, to preserve the house.