The 25-acre Hillwood Estate in Washington's Forest Hills just above Rock Creek Park - are being opened to the public. Probably in the fall the house with its noted Russian Collection will be a museum.

Included as well as the gardens are the greenhouse, with its 5,000 orchids (interesting chiefly in the Christmas-to-Easter season) and the dacha - a wooden resort cottage such as the Russians retreat to for weekends - containing some of the Russian collection.

After Mrs. Post's death the house and gardens were given to the Smithsonian Institution, which concluded after some months that it should not keep the gift, because, according to the Smithsonian spokesman, the rigid restrictions in Mrs. Post's will would make operating costs exceed the $10 million endowment. Last April, the property title reverted to the Hillwood Foundation, which was not bound by all the restrictions and is free to charge admission to augment the endowment.

As Smithsonian property, it was not on the tax rolls, but its tax situation has not yet been made clear as foundation property. Rodger Mastako, director, said that however that is decided, the place will be open to the public. It is evaluated, for tax purposes, at $3.3 million, the second-highest assessed residential property in the city.

From the house portico you look south to see, far in the distance, the Washington Monument rising above the trees that otherwise form the horizon.

This roomy sweep of lawn is bordered by a rhodedendron walk. At the center you descend steps, with walks leading to smaller gardens, the whole laid out by landscape architect Perry Wheeler.

Toward the bottom is a Japanese @ garden, with stepping stones across an improvised stream.

There are various dwarf conifers and non-dwarf rocks and stone garden lanterns.

Swinging back to the left there is a pet cemetery, the entrance guarded by stone poodles holding carved baskets of flowers. Then up to the dacha, which looks quite exotic, through the cutting garden (which has a row of red peonies coming into bloom) and down to the greenhouse.

This looks inviting and miniature, though by the ordinary gardener's standards it is huge. The entrance court of the greenhouse has a little pool and staging for the handsomest orchids that happen to be in bloom. Americans like to throw pennies in the pools of gardens they visit, and it does no harm here since there are no fish (copper poisons fish) and maybe Mastako will become rich.

Walking on past the main entrance of the house (noticing the agreeable copper eagle for a weather vane) you come to the rose garden. This is a fine sunny place, bordered by arbors made of 8-inch timbers, with roses spread out before you. Back toward the house from his garden, reached by a little walk, is the French Garden, paved with stone for water to glisten in, as it runs the length of garden from a fountain. This device is Moorish, but the simplified planting of box bushes clipped low and in curlicews like embroidery, is about as French as you can get in a garden.

It is very pretty, and this type of gardening is ideal for people with limited space, but who want something that looks good all year, and who are not much tempted by gardening in the first place. Such gardens are the easiest to maintain, and avoid the shabby disreputable look of gardens where weeds intrude among the daffodils.

The garden is full of small sculptures that Mrs. Post in her lifetime liked to have about her, since they were given by friends and brought her happy memories.

People who cannot walk down stone garden steps, I have always thought, should not attempt them, and such people are certain to break their necks anyway, so nothing much is accomplished by painting the edges of the stone white. It ruins the effect of the stone, and in any case nothing short of a sedan chair and a 10-foot wide concrete ramp that drops one inch per 50 feet will get some people through a garden safely.

But of co urse it worries people responsible for public safety, that someone might fall. I understand their concern, I merely say that stone should not have white stripes painted ont it.

Trials tours, by invitation, have been conducted this week, and worked out well, so here is how you see the garden:

Written reservations are required. You write the Hillwood Museum Gardens, D.C. 20008, preferably two weeks before the day you want to go, and send $2. They confirm the date and hour (alternate dates may save correspondence) and you show up at the gate and are admitted. Tour days are Mondays, Wednesday, Fridays and Saturdays, and four tours a day are possible, at 9:30 and 11:30 a.m., and 1:30 and 3:30 in the afternoon.

The property is in the middle of some of Washington's most desirable real estate and the gardens were never designed for heavy traffic. Each tour will be limited to 25 people. If you need special help (wheelchair, for instance) this should be mentioned when you write.

When you get there you go to a pleasant small auditiorium where they run off a show of color slides explaining who the Posts were (Grape Nuts, Postum, General Foods) and a few personal things about Mrs. Post.

You then are shown the several acres of gardens, the dacha and the greenhouse and may wander about the grounds or visit the souvenir shop.

It is not the kind of garden that a fanatical gardener must go to see, but it is a pleasant place and a valuable gift to the public. When the house is also open (the tour will then be $3 for both house and garden) it will be an even greater attraction.

Even without the house, the visitor will always find an orchid here, a dwarf spruce there, a vista over yonder, to please the eye.

Hilwood is the only one of the following nine highest tax-assessed residences in the District of Columbia to be open to the public. (Distric properties are now assessed by the District tax assessors office at the price the assessor believes to be the true value. However when properties are sold, the price is not necessarily the assessed value. It may be higher or lower, depending on the seller and the buyer. The Rockefeller property, for instance, is currently up for sale with a $8 million asking price, despite a tax evaluation of $2,535,437. Firenze House was recently sold to the Italian Embassy by Polly (Mrs. John) Logan for $4,335,000 despite an evaluation of $2,144,359. Not all the properties have been reassessed this year which of course could change the evaluation.) The top valued properties, in descending order of evaluation are:

3901 Reservoir Rd., owned by the Archbold Investment Co., valued at $5,026,803;

Hillwood, 4155 Linnean Ave. Nw, $3,300,00, owned by the Marjorie Meriweather Post Foundation of the Distirct of Columbia;

2500 Foxhall Rd. NW, residence of Nelson Rockefeller, $2,535,437;

3029 Klingle Rd., (Tregaron, also once the home of Marjorie Merriweather Post, when she was married to former Ambassador Joseph Davies), owned by William E., Joseph D. and Willard E. Tydings, Robert Grosjean and E.F. Broun, $2,508,818;

2816 Battery PI., trustees, Thomas J. Owen and Donald Glen, Chain Bridge Road Home Owners Corp., $2,214,087;

Firenze, 2800 Albermarie St. NW, the embassy of Italy, $2,144,359 (likely soon to be off the tax roles because of the change in ownership);

2401 Foxhall Rd. NW, owned by Mr. and Mrs. David Lloyd Kreeger, $2,110,753;

3201 New Mexico Ave. NW, owned by Charles C.Glover Jr., $2,000,342;

1801 Foxhall Rd, NW, owned by Elinor Brady, $1,863,152.