Preparations for the construction of a new three-storey 20,000sq ft (1,858sq m) spa at Sun Valley Resort have begun, adding to the existing Sun Valley Lodge in Idaho, US.

According to the design-review application – to be discussed later this month by the Sun Valley Planning and Zoning Commission – the 141,100sq ft (13,108sq m) lodge will be expanded to 172,487sq ft (16,024sq m) once the project is completed.

The plans for the spa comprise 15 treatment rooms for individuals and couples, steam and sauna facilities, relaxation lounges, a yoga studio and a large fitness centre. The third floor will house four new guest suites – two that are approximately 1,100sq ft (102sq m) and two that are 600sq ft (55.74sq m).

The 485-bedroom hotel's existing heated outdoor pool will be reconstructed, without many changes to its appearance. A poolside cafe will feature on the lodge’s expanded deck.

The lodge’s existing spa area has been temporarily relocated and the Lodge Dining Room has been closed so that construction crews can work on the interior and exterior of the northerly wing of the lodge.

While the lodge will remain open through the summer as the spa’s construction progresses, the resort hopes to shut up shop from September 2014 to June 2015 to allow for the remodelling of the entire structure.

Development plans also include remodelling the family bowling alley in the lodge and adding a kids-oriented food service.

The project will be overseen by Sun Valley’s usual duo: Boston design firm Frank Nicholson and Ketchum-based architects Ruscitto, Latham and Blandon. Previous collaborations between the two companies for Sun Valley include the Sun Valley Pavillion (2008) and Carol’s Dollar Mountain Lodge (2004).

Privately owned by the R. Earl Holding family, Sun Valley Resort is an all-season mountain resort which offers skiing, snowboarding, nordic skiing, golf, mountain biking and summer recreation.

The R. Earl Holding family also owns of Sinclair Oil Corporation. Sister properties include Snowbasin Ski Resort; Grand America Hotel; Little America Hotel and Towers, Salt Lake City, Utah; Westgate Hotel, San Diego, CA; Little America Flagstaff, AZ; Little America, WY; and Little America, Cheyenne, WY.