RESIDENTS of one of Scotland's richest streets are fighting plans to save a mansion built by one of the country's most celebrated architects.

Morar House, designed by William Leiper, sits opposite Charles Rennie Mackintosh's celebrated Hill House in Helensburgh.

Despite both properties being hailed as heritage gems, their fortunes could not have been more different.

Mackintosh's creation is looked after by the National Trust and attracts more than 40,000 visitors a year while its neighbour, "Hell House" – as it has been dubbed by locals – has been left to rot.

Now a battle has broken out between residents and a conservation architect who wants to save the building but only if he is allowed to convert part of it into flats and build more houses in the grounds.

Morar House, an 11-bedroom red-tiled mansion with commanding views of the Firth of Clyde, was built by Leiper for a Glasgow shipping family.

Following his successes with Dowanhill Church of Scotland in Glasgow, Templeton Carpet Factory on Glasgow Green and the interior of Glasgow City Chambers' Banqueting Hall, Leiper established a reputation as a leading Gothic architect.

However, while Hill House has been proposed as a World Heritage Site, the Leiper house was allowed to decay until Edinburgh-based architect Lorn Macneal bought it for £322,000 at auction last October.

He has now applied to Argyll & Bute Council to convert the interior into 11 flats and build a house and two mews cottages in the grounds to off-set the estimated £1.5 million cost of saving as much of the mansion as possible.

Mr Macneal's previous projects include Falkland Palace in Fife, the Pineapple House near Falkirk and Stevenson House in East Lothian.

He said: "At the moment we have Hill House on one side of the road and Hell House on the other. I want to create something that is in keeping with the landscape setting of Upper Colquhoun Street and befitting a conservation area."

However, his plans have run into some opposition from residents of a street where properties fetch up to £800,000.

Helensburgh Community Council has written to Argyll & Bute planning authorities expressing concerns that the proposals will lower the architectural importance of the A-listed building, which features on Historic Scotland's Buildings at Risk Register.

Other residents of Upper Colquhoun Street, which has 18 houses, have complained about invasion of privacy, an increase in traffic and damage to the conservation zone.

Retired businessman John Ashworth, whose property borders Morar House, said: "The proposed development as it stands would constitute a gross intrusion into my privacy.

"The plans have been designed to protect the facade of the property the public will see from Hill House but it means the rear wall of the new development will run for some 60 yards, just four metres from the boundary of my property. I don't believe that is in-keeping with a conservation area."

Helensburgh Civic Society has called on the council to seek the views of Historic Scotland before making a decision in order to head off any costly delays in resolving the issue.

Much of the interior of the building has already been lost to dry rot, damp, decay and vandalism and Mr Macneal has warned that if he is refused planning permission the project will have to be abandoned, fuelling fears the Leiper house will be demolished.

He said: "This is a major project. I cannot go on putting in good money after bad. If permission is refused I will have to take the hit and move on. I'm not trying to put a gun to anyone's head – it is simple practicalities."