Not every house is open every day, so be sure your plan for seeing the Treasure Houses of Derbyshire takes into account individual schedules. Even then it is wise to check opening dates and times by telephone once you are in England. The disappointment of arriving at Hardwick on a Tuesday (as I once did) is enough to enrage the most ardent Anglophile.

In addition, common sense helps. Since Chatsworth is open every day (March 23 to Oct. 26) and is easily the star in Derbyshire's sky, miss most of its traffic by avoiding the weekends when the Brits troop in with kiddies, mums and a hearty picnic spread out right in front of your viewfinder. In the same spirit, skip the days that cater to special interest groups by asking ahead. (This year they are May 18, July 2, July 18-20, Aug. 30 and 31 and Oct. 3-5.) And arrive early enough to grab a shady spot in the car park and the first glimpse of a few empty rooms.

Buy the official brochure about the house. It will usually cost just a pound or two and will serve as a guide, helping you plan your day. And while you're spending money, pay to see the extra rooms not included on the regular round. For a few more pennies, you'll have an opportunity to see something unusual.

Even if you don't have lunch or tea at the house you're visiting, look in at the old stable, or kitchen, or whatever has been converted into the restaurant. It's intriguing to see an ancient room put to modern use.

Take a raincoat, boots and an umbrella. Even if the day is clear, the ground may be wet and a shower will loom on the horizon.

And take it easy. Take advantage of those British customs, morning coffee and afternoon tea. Visit no more than one house a day, or they'll begin to mingle in your mind and fade from memory. Haddon Hall, Hardwick Hall, Chatsworth House and Kedleston Hall are very different from one another.

GETTING THERE: Despite the fact that relatively few Americans have discovered it, Derbyshire is easy to reach. Regular flights from London -- and now New York -- serve the Manchester airport, a growing terminal that is about an hour and a half by car from the town of Matlock. There is also frequent train service to Derby from London's King's Cross station; the trip takes between two and three hours. Car rentals can be arranged at the Manchester airport and the Derby train station. The drive from London to Matlock is about five hours.

GETTING AROUND: Unless you have endless hours to spend waiting around at train stations and bus stops, the only feasible way to tour the Treasure Houses of Derbyshire is by car, with a few forays on foot. Half the fun is getting "lost" on improbable roads. But buy good maps, because serendipity is most amusing when you know your way home.

WHEN TO GO: The visiting season for these houses is limited. In the spring, there are fresh paint and new slipcovers for the annual opening, and gardens are at their most flamboyant. On the other hand, the national weather forecast all too often calls for "bright patches," a BBC euphemism for intermittent rain. In the fall, homeowners, houses and even flower beds are sometimes tired, but the weather has a way of settling into long stretches of sunny days. Take your choice, but try to avoid the waves of crowds that crest in the summer.

THE HOUSES: Admission fees to the Treasure Houses range from two to three pounds for adults, half that for children.

*Haddon Hall, phone Bakewell 2855, is two miles southeast of Bakewell on Route A6. It's open from Good Friday or April 1 (whichever is earlier) to the end of September, 11 a.m. to 6 p.m. In April, May, June and September, the house is open daily, including bank holiday Mondays, although it is closed all other Mondays. In July and August, it's also closed on Sunday. Lunch and tea are served in the Stables Tearoom.

*Hardwick Hall, phone Chesterfield 850-430, is nine miles southeast of Chesterfield near Doe Lea on Route A617 (near Route M1). It is open March 29 to the end of October, on Wednesday, Thursday, Saturday, Sunday and bank holiday Mondays, 1 p.m. to 5:30 p.m. (or sunset, if earlier); last admission to the house is at 5 p.m. The garden is open every day during season, from noon to 5:30 p.m. The Great Kitchen serves lunch from noon to 2 p.m. and tea from 2:30 to 5 p.m.

*Chatsworth House, phone Baslow 2204, is four miles east of Bakewell on Route A623. It is open March 23 to Oct. 26, daily from 11:30 a.m. to 4:30 p.m. Lunch and tea are served in the stables.

*Kedleston Hall, phone Derby (0332) 842-191, is open from April 27 to Aug. 31, on Sunday, bank holiday Mondays and Tuesdays, and on the Monday and Tuesday following Easter, from noon to 5:30 p.m. Light lunches and tea are served.

WHERE TO STAY: It's wise to make reservations in advance. There are a number of hotels not far from the Treasure Houses. Among those that accept major credit cards and offer private baths with all rooms are:

*Rider Hall, Matlock, Derbyshire DE4 5JU, phone Matlock (0629) 2795. This is a lovely Elizabethan manor house in a walled garden, with friendly service, excellent food and a good wine list. There are 11 rooms, and rates range from $90 to $135 double, including breakfast.

*Cavendish Hotel, Baslow, Derbyshire, DE4 1SP, phone Baslow (024-688) 2311. This is a sturdy old inn near Chatsworth , with comfortable accommodations, a picturesque lounge and bar (with brick floor) and peaceful surroundings.

There are 23 rooms, with rates at about $80 double.

*The New Bath Hotel, New Bath Road, Matlock Bath, Derbyshire DE4 3PX, phone Matlock (0629) 3275. This busy hotel blends Georgian and modern-motel styles of architecture . It is set in a park and offers tennis, swimming, a sauna and conventional food. There are 56 rooms, with rates at about $80 double.