The planned glass and stone extension at the Church of St John the Evangelist in the West End would house an expanded hall, exhibition and performance spaces, as well as offices and meeting rooms.
Church leaders have set September as the target start date for building work which is expected to last around 18 months, meaning the high-profile place of worship misses only one festival as a key Fringe venue.
St John’s – a category A-listed building – was designed by the 19th century architect William Burn and consecrated in 1818.
It has remained largely unaltered since the construction of its church hall in 1916, but the extension will dramatically change its appearance from the Princes Street Gardens end.
Charlotte Bray, St John’s appeals manager, said nearly £2.4m of the overall cost had been successfully raised from church members, charitable trusts and donors.
The funding bid has now been opened to members of the public, who it is hoped will help provide the “final million” needed to ensure building work can get under way.
Ms Bray said: “This is about improving a space that community groups use – it’s part of our commitment to the city around us.
“This is the final leg of the marathon. A charity abseil with Lord Provost Donald Wilson at the weekend was the start of the public phase of the fundraising effort.
“The aim is to build on the amount of money that we have already raised to get us to the target. It’s the final million we’re looking to raise now.”
Ms Bray said the overhaul was aimed at respecting and enhancing one of the most significant pieces of church architecture in Edinburgh.
She said the extension would enable the St John’s community to make excellent use of the church space while overcoming the existing building’s physical limits.
“It feels very energised at the moment,” she said. “We’ve been talking about this project for quite a long time and to finally feel that this vision is becoming a reality is really quite exciting.
“Obviously we’re nervous as well [about raising the remaining funds]. But we’re not a static church. We’re a living building and I think that’s what people will recognise.”
Community leaders said they had yet to reach a position on the project but remained open-minded.
Jock Miller, chair of West End Community Council, said every application should be assessed on its own merits.