The 300-member Brighton and Hove Progressive Synagogue is selling the rear section of its Archie Fay House to help fund the shul's redevelopment project.
Vice-president Prue Baker said the aim was to provide "a fully accessible, energy efficient and attractive synagogue for our growing membership and a Progressive Jewish cultural centre for Sussex and the south coast."
Building work is expected to start in October and take a year to complete. During construction, Shabbat morning, festival services and cheder classes will be held in the Ralli Hall Community Centre, with Shabbat evening services held in members' homes.
The shortfall will be met by fundraising activities such as a members' cabaret evening, which earned almost £1,000.