The castle, which is situated on the Beauly Firth, west of Inverness, purchased and developed nine extra properties on the estate as part of the expansion funded by HSBC.
This includes eight luxury residential lodges, capable of housing an additional 76 guests, which were completed in November 2015.
The ninth building is set to become a restaurant called Table Manors, due to open prior to Easter, and the owners are looking to build on their solid performance last year.
Turnover grew by a third in 2015 and further growth is anticipated as awareness of Achnagairn grows internationally and the restaurant becomes operational.
Gillian Lacey-Solymar, co-owner of the castle's owner Perfect Manors, said: “The HSBC funding has allowed us to transform our offering.
“The additional accommodation makes us one of the few wedding venues in Scotland that can have up to 150 guests staying overnight on an exclusive use basis.
“HSBC’s team has been hugely supportive of our plans for Achnagairn Estate, which is now a five-star luxury destination to match any in the UK.”
Susan Rowand, head of business banking for HSBC in Scotland, added: “Gillian and Michael’s vision for Achnagairn estate is highly ambitious and extremely successful.
“Not only has Achnagairn become hugely popular in the UK, they are seeing more visitors come from abroad and wedding parties booked from as far afield as Shanghai.
“The addition of 40 new bedrooms makes Achnagairn Estate internationally attractive and we’re very excited to be a part of this journey and to support further growth.”