A scheme to convert an historic mansion, designed by a celebrated architect, into two apartments should be rejected, according to a civic watchdog.

And Helen Kidman, the chair of Ilkley Civic Society, says the plans to transform Grade II-listed Heathcote, in King’s Road, would “compromise” its importance as one of the town’s most significant villa residences.

The Sir Edwyn Lutyens-designed house, dating from 1906, was sold by NG Bailey at the beginning, after serving as offices for the firm since 1958.

A planning application, submitted by legal firm Cobbetts LLP, is seeking permission from Bradford Council to change the use of the main house to two residential units, carrying out minimal alterations to the internal layout, and make alterations to the nearby cottages, converting them into three homes.

But the scheme has already come under attack from Martin Lutyens, the great nephew of Sir Edwyn and chairman of the Lutyens Trust, who said changes would cause an “unacceptable harmful impact” and English Heritage has also voiced disapproval.

In her letter the council, Mrs Kidman describes Heathcote, which was on the market for £2.5m in January, as “an important listed building in Ilkley and probably the most significant of the town’s many villa residences”.

“In that respect, the whole property, including the outbuildings, grounds and associated structures, are important,” she added.

“We consider that every effort should be made to keep the house in single occupation.”

Mrs Kidman says the civic society has concerns about several aspects of the proposed conversion, including plans to convert a walnut-panelled billiard room into a kitchen/diner.

Meanwhile, Yorkshire Gardens Trust says it has no opposition to the planning application, as long as the gardens are maintained to a high standard.

In a letter to the council, the trust’s Val Hepworth says Heathcote is one of only three examples in the north of England of a Lutyens-designed house with a Gertrude Jekyll-styled garden.

“Heathcote is a prime example of the ideal of a ‘Lutyens house with a Jekyll garden’ that became an important contribution to the English way of life,” Mrs Hepworth said.

“The Yorkshire Gardens Trust hopes that the future will see that this ideal continues here in Ilkley.”

No date has been set for a decision on the planning application.