Work has begun on a £500,000 extension to the Bridge Lane Beth Hamedrash in Golders Green.
Congregant and philanthropist Leo Noe laid the first stone at a ground-breaking ceremony on Sunday.
The new building has been 10 years in inception, due to planning issues and redesigns. As "green and ergonomic as possible", it will incorporate a day centre for the elderly.
Beth Hamedrash trustee Jonathan Wachsman said: "The new synagogue will make an enormous difference. There will be more space and functions can run side by side."
It is hoped the extension will be completed by next Rosh Hashanah.
