The 36,000-square-foot refurbished building in the Old Street area will offer space for up to 80 charities when it opens in the spring.
It will double the office space operated by social landlord Can Mezzanine and brings the total number of organisations it provides with low-level rents to about 200.
The £10.5m transaction was funded largely by social lenders Triodos and Charity Bank.
"That Can Mezzanine has such a solid social business model, which allows us to push through this transaction in the worst financial climate, not only makes us very proud, but signifies the power and potential of growing social enterprises," said Andrew Croft, chief executive of Can, the charity behind Can Mezzanine.
"Our model is an attractive proposition for investment because, as a charity-based social enterprise, we don't maximise shareholder profit but reinvest our surpluses to create more public benefit."
He said he intended to continue to roll out the Mezzanine model across the UK, and that the buildings saved millions of pounds a year for charities in fees for accommodation and services.