Is it really necessary to destroy this garden?

There are plans to build eight executive-style homes in the perfectly preserved grounds of Downe Hall in Dorset. Anna Pavord is appalled

Knights in shining armour are always in wretchedly short supply but if there is one cruising in the vicinity of Bridport, Dorset, would he please go post-haste to Downe Hall, an 18th-century mansion sitting on the side of Coneygar Hill, and make an offer for the place. Its owner wants to get rid of it, although it has never appeared in the pages of Country Life or indeed anywhere else on the open market.

Now this is not some troublesome wreck surrounded by an industrial estate. It is an elegant house, not impossibly large, and sits in the middle of gardens and woodlands so extensive you can scarcely believe that within a couple of hundred yards is Bridport's main street, another treat of 18th-century architecture.

Even more surprising, Downe Hall with its 14 acres of garden and surrounding acres of pasture and woodland has staggered through the infilling mania of the last 50 years to survive virtually intact in its design and layout since William Downe first moved here to take the sea air in 1789. The perimeter walk, a great feature of villa gardens of the period, is still protected with Portugal laurel, box and yew. There are some magnificent trees, including two enormous plane trees, rare in these parts. Some of the beech and lime date from the time the grounds were laid out in the late 18th century.

From the terrace along the south front of the house, you still get what a sales notice of 1837 described as a "bold and extensive view of the vast ocean", framed between folding cliffs. The terrace itself is the work of Edward Prior, an Arts and Crafts architect and pupil of Norman Shaw. It has great period charm, with wisteria coiling around the retaining walls and wide, semicircular flights of steps connecting the different paved levels.

Rooks still clatter about in the trees here and a pungent whiff of badger hangs in the air. Against all the odds, this house and its setting exist in a serene, untouched bubble. But at the end of this month, barring some "new material consideration", West Dorset District Council will vote on whether to allow a local firm, C G Fry and Son to build eight executive- style homes in the grounds of Downe Hall, while converting the house itself into five flats. Eddie Fry, dubbed "the Prince's Builder" (he is the developer of Poundbury, the Prince of Wales' new-build project on the outskirts of Dorchester, Dorset's county town) is acting as agent for Downe Hall's owner, Mrs Morse-Letheren. He has persuaded local planners that this is the way to ensure a future for the house, listed Grade II*.

Mr Fry's special relationship with the local planning authorities, made clear when I talked to Des Derrien, Director of Planning and Environmental Services at the West Dorset District Council, was built up in the wake of his successful development in the Dorset village of Abbotsbury. If development has to take place, the Council feels he is likely to make a better job of it than anyone else in the locality.

Does development have to take place? This is one of the questions that has been asked from the beginning by Bridport resident Catherine Searle, who has fought harder than anyone to find ways of preserving the entity of Downe Hall. Could not the council delay giving planning permission until the property had at least been tested on the open market? Mr Derrien voiced fears about "unscrupulous developers" getting their hands on the place, but there are laws to prevent unsuitable development. His department can enforce them.

Could not the council seek an independent assessment of the economics of the site? "Enabling development" is sometimes granted to generate the finance necessary for a charitable trust to preserve a listed building, but as Anthony Jaggard, chairman of the Dorset Natural History and Archaeological Society has pointed out, this is not such a case. It is, as he said, "a speculative investment" not unnaturally geared to maximise the greatest possible return. But is it strictly necessary to build as many as eight houses to provide the funds to convert the house into five flats, which can themselves be sold at a profit? It is not too late once again to ask the question: is a housing development on the scale envisaged at Downe Hall the best possible way of ensuring its future?

The Georgian Group thinks not. The Victorian Society thinks not. The Garden History Society most emphatically thinks not. David Lambert, case officer for the Society says Downe Hall is "the saddest case" that has come his way over the past year. "The structure is so little changed from the time it was laid out", he explained. "The perimeter belt is mature and unbreached by modern development. The late 18th-century trees in the park give it a very special character. The perimeter walk is intact and the house itself amazingly unspoilt by 20th-century additions."

When you look at the plans, it is immediately obvious that the greatest harm to the setting is caused by the four houses proposed for the foreground of Downe Hall, two on either side of the presently unbroken sweep of grass and trees. If these could be done away with, then the house and garden would be very much less jeopardised than they would be under the present plans. There is a pretty lodge house which has been empty for the last 17 years and a stable yard built by Prior that could be converted instead, if the need for housing in Bridport is thought to be so great (the local paper, the Bridport News advertises a selection of 48 houses already for sale in the town). This is a compromise - the Downe Hall bubble would still be broken - but it is a realistic one, if not quite as financially rewarding for the property's owner and agent.

At the meeting, now very soon, that finally seals Downe Hall's fate, members of West Dorset District Council's planning committee may like to remind themselves that it was they who first identified the special importance of Downe Hall, whose wooded grounds lick like a tongue down into the centre of Bridport. In this town, which already has special planning status as a conservation area in an Area of Outstanding Natural Beauty, they decided that the Downe Hall site merited another girdle of protection as land of local landscape importance. Under Policy L4 of the local structure plan, this stipulates that "development proposals which would harm specific features and qualities of local importance will not be permitted."

Do the 40 trees that must be felled to accommodate eight houses count as specific features? Does it matter that a double garage has been positioned astride the presently unbroken perimeter walk? Or that the massive yew hedge and topiary to the northwest of the house will be buried under another garage block? In short, have we learnt nothing from the mistakes of the last 50 years? Time is running out for Downe Hall, but with strength and vision on the part of the planning authorities, it could still emerge as a building saved without a garden lost.