The Grade II listed Georgian house – adjoining 80 Derngate – has been open to visitors in its current guise since 2003, following a £1.4 million restoration project.
The 78 Derngate Northampton Trust has been managing the building since 1998 - after it was given to them on a 100-year lease from the borough council.
READ MORE: Before its time historic Art-Deco house is a Northampton treasure not to be missed
Now the trust has launched an ambitious fundraising appeal to extend the atrium area - the open-roofed entrance hall - to make way for a bigger shopping area and a new tea room.
It has already been granted £200,000 from Northampton Borough Council, but the trust believe they can match fund the sum.
The aim is to secure the monry by the summer and bosses at 78 Derngate are asking supporters to donate £100 each.
For each £100 the donors’ names will be recognised on the engraved glass panelling located in the new atrium.
Donations can be Gift Aided, paid for at reception or online through the secure portal here.
The new extension will also make way for better disabled access, a bigger storage area andr reception and will free up the pantry in 78 for visitor viewing - which bosses say will increase revenue and reduce running costs.
Scottish architect Charles Rennie Mackintosh was asked to work on the terraced house in 1916 by Northampton-born model maker Wenman Joseph Bassett-Lowke - though Mr Mackintosh did not ever visit the house in person.
The house’s later history saw it pass out of the hands of the Bassett-Lowke family. By 1964, it was the base for Northampton High School.
When the school sold the site, a campaign was launched, backed by the Charles Rennie Mackintosh Society, to enable its preservation and restoration; a dream which came true thanks to the work of the 78 Derngate Northampton Trust and financial support from Heritage Lottery funding.