The total figure is split between six of Unique Venues of London’s 81 sites including Freemason’s Hall, the Foundling Museum, LSO St Luke’s, Gibson Hall, London Museum of Water & Steam, and IWM London.
Moya Maxwell, chairman of Unique Venues of London, said: "2014 is a special year for Unique Venues of London, as we celebrate our 21st anniversary and launch new branding to represent the consortium’s development.
The multi-million pound investment will see the IWM London open new event spaces on 19 July, including a roof terrace and daytime rooms.
In addition, the Foundling Museum has updated its Introductory Gallery, while Freemason’s Hall has refurbished its Grand Temple events space.
On 8 July, Unique Venues of London will unveil a full brand relaunch following this year's investment.