The terraced property in Northampton, known as 78 Derngate, is the only home in England created by Mackintosh, who was seen in Europe as a leader in avant-garde art and design.

New gallery space has been added to an adjoining building.

Liz Jansson, house manager, said it would help keep the tourist attraction open for “generations to come”.

Designed and re-modelled by the Glasgow architect in 1916, 78 Derngate is the only place where his style can be seen in its original setting.

The designer of Hill House in Helensburgh, the Glasgow School of Art and MIss Cranston’s tearooms in Sauchiehall Street, Glasgow, he was invited to remodel the Georgian house by the renowned model-maker Wenman Joseph Bassett-Lowke in time for his wedding in 1917.

The house was bought by Northampton High School for Girls in 1964 and used as a school.Local people united with members of the Charles Rennie Mackintosh Society to campaign for its preservation and it opened to the public in 2004.

The visitor attraction has capacity for just 30 people but the trust that runs it also owns two neighbouring properties.

The £950,000 two-storey glass extension means there is now twice as much gallery space as before and a new shop and education centre.

The gardens have also been extended and it is hoped the work will help increase visitor numbers.

Ms Jansson said the space was “fantastic” and worked “seamlessly” with the original buildings. “It is like it has always been here,” she said.

The project was paid for from the attraction’s own fundraising and £400,000 from the £25 million given to West Northampton-shire Council from the government’s Town’s Fund.

The new gallery has been in use for several weeks and will be officially opened with a ribbon cutting ceremony on Friday.