The Victorian Society said Spenfield House is one of the best preserved Victorian homes in northern England
Campaigners are fighting plans to turn a Victorian Grade II*-listed building into flats.
Owners Round Strategies want to convert Spenfield House in Headingley, Leeds, into six apartments and a studio flat.
Leeds Civic Trust and the Victorian Society held an emergency meeting on Monday night to discuss the plans.
They claim the property is one of the ” best-preserved Victorian homes in the north of England” and want it turned into a museum.
Clive Woods, from the trust, said more than 70 people attended the meeting and they would now form a “Friends” group to put forward alternatives for the site.
The Victorian Society said the building had “exceptional interiors”
He said: “It’s the best Victorian house of its type in the north of England, so to build in the garden and split it into apartments is unbelievable.
“It might be in Headingley, but it’s a heritage icon and they want to damage it irreparably. We want to make sure planning permission isn’t given.”
The house, just off Otley Road, was built between 1875 and 1877 and is in the Weetwood Conservation Area.
The Victorian Society said the building has “exceptional interiors” which will be lost if plans are approved.
As well as converting the property, the owners want to build eight terraced houses in the grounds.
Agents DPP were contacted for comment but did not respond.
A similar scheme put forward in 2011 was given the green light by planning chiefs but never built.
Round Strategies put forward its most recent proposal earlier this year.