The London-based starlets beat a trio of the country's most promising practices with a 'highly innovative' carbon-neutral scheme to bag the prestigious project to create a 20-25 bedroom hotel on the rural plot outside Chorley, near Preston.
Judge and former RIBA president George Ferguson said the jury was 'spoiled for choice' by the quality of the four finalists but eventually plumped for Softroom ahead of Featherstone Associates, Riches Hawley Mikhail Architects and baca.
Ferguson said: 'Softroom gave us an intelligent and environmentally aware solution that rises to the challenge of the site and its history - and most importantly has the potential to become a beautiful contemporary country house hotel of which Lancashire can be truly proud.'
The practice's scheme will sit on the site of the original Croston Hall, which was built in 1857 for the de Trafford family, but was flattened in 1964.
Although the planners have yet to see the design, Martin Ainscough, the client, is hopeful that the scheme will be well-received.
He said: 'There is a long way to go before it gets built, but we are confident that we have a very fine design and a strong team to take things to the next stage.'
The RIBA-organised competition attracted 66 expressions of interests from around the world.