As part of the investment, Amazing Retreats has bought the 12-room Plas Rhianfa in Anglesey and the nine-room Kinettles Castle near Dundee.
Meanwhile, extensive renovations are planned across the portfolio, including £750,000 for gardens and outdoor space.
Refurbishment includes the addition of eight catering kitchens and more than 60 bedrooms, as well as creating banqueting space at Morley Old Hall, Beau Castle, Ackergill Tower Plas Cilybebyll, Stanbrook Abbey and Tempsford Mill.
Leisure facilities including five swimming pools, six gyms and spas, two tennis courts and four games rooms will also be added in 2011.
A £1.5m refurbishment is taking place at Spitbank Fort in the Solent, due to open later this year, that will transform it into a nine-room luxury venue with function space for 50.
Amazing Retreats executive chairman Mike Clare said: "The multi-million pound investment underlines our commitment to all our guests, no matter what the purpose of their visit, by consistently developing and improving our product. As a result of this investment we expect 2011 to be a busy year for us, with new properties becoming available and new facilities opening at existing locations."
Ackergill Tower, a 15th century Scottish castle will open additional conference space by December following a stables conversion. While Plas Cilybebyll, an 18-capacity manor house in Swansea is turning disused garages into an Orangery dining facility for 120 - due for completion by May.
Beaucastle, a Venetian Gothic-style house in Worcestershire, which sleeps 14, is transforming a derelict coach-house into a dining facility for 120 by September. And Morley Old Hall in Norfolk is converting two barns into additional conference and dining space, also due for completion by September.