Anglesey Abbey in Cambridgeshire and Treasurer's House in York are now available for all manner of events, from gala dinners to product launches.
Dating from medieval times, Treasurer's House boasts the Great Hall, a versatile space for presentations and drinks receptions, while Anglesey Abbey is known for its landscaped gardens and Scandinavian-style visitor centre.
The additions will mean that clients can now expect a higher acceptance rate for hosting events at unique and historic venues.
Three other venues - Ham House in west London, Claydon House near Buckingham and Felbrigg Hall in North Norfolk - are due to be added to the portfolio this spring.