South Hampstead Synagogue chairman David Turner has pledged that the shul will not borrow money to finance its rebuild, even though the cost is likely to rise beyond the projected
£12 million.
He also promised that there would be no "formal levy" on members to meet any additional outlay.
Although unable to give "a reliable figure" on the actual cost, Mr Turner insisted that the long-awaited redevelopment remained within the congregation's means and would not leave "any liability to future generations".
Camden Council granted planning permission in December 2014 for a new complex on the synagogue's Chalk Farm site.
A substantial contribution has been offered by one anonymous donor to what will be the most expensive redevelopment scheme undertaken by a United Synagogue congregation. Further pledges totalling around £5 million are understood to have been made.
Unusually, the freehold of the existing site is owned by a charitable trust rather than the US.
The JC understands that the US has expressed interest in buying the freehold for around £2 million - although neither party will confirm this.
US community services director David Kaplan said the development of South Hampstead was "a hugely significant project which will further strengthen this vibrant and energised community. An initiative of this scale requires the utmost care in planning and execution."
Mr Turner said the community remained "on track" to begin rebuilding in May.
It had intended to hold regular services at the Marriott Hotel Regents Park, but the venue will be unavailable for several months because of renovation.
For the interim, alternative venues were being finalised.
The hope was that building work would take 21 months but "we have factored for two years".
He added that the shul was also close to submitting planning permission to the council for a local eruv.