A trust protecting the work of celebrated architect Edwin Lutyens says plans to split a historic Ilkley mansion into two apartments should be rejected.

The Lutyens Trust told Bradford Council that present proposals for the grade II*-listed King’s Road property, its cottages and grounds, would have an “unacceptable harmful impact”.

Great nephew of Sir Edwyn and chairman of the trust, Martin Lutyens, met the applicants behind the plan at Heathcote in April, along with the trust’s architectural advisor, Dr Mervyn Miller.

The trust has now sent a number of concerns about the proposed internal alterations to the council, in response to the planning application submitted recently by legal firm Cobbetts LLP.

Dr Miller said: “Heathcote is known internationally as one of the finest ensembles of an Edwin Lutyens House and a Gertrude Jekyll Garden, and its ‘gold standard’ historic significance deserves appropriate conservation of the highest quality.”

The Lutyens-designed house, dating from 1906, was sold by Ilkley-based company NG Bailey at the end of last year, after serving as offices for Baileys since 1958.

Cobbetts 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.

The Lutyens Trust acknowledged that the current proposals would reinstate Heathcote and its ancillary buildings to residential use, and said it brought a gain in their collective significance as a designated heritage asset.

But in practice, said the trust, it felt any gain would be more than outweighed by an “unacceptable compromise” of the essential qualities of spacial planning and sequential experience of the building as Lutyens intended.

English Heritage has also expressed concerns.

Historic buildings and area adviser Kathryn Gibson said English Heritage could not at present support the current applications as it had not been demonstrated that the proposals would deliver a long-term sustainable future for Heathcote that would outweight the harm to the significance of the building.

Ilkley Parish Council’s plans committee, however, has recommended that Bradford approves the plans.