The winning scheme - the House at Three Seton Mains in East Lothian ( pictured) - was announced at a ceremony at the RIBA's Portland Place headquarters at the end of last week.
Last year's winner, Nord Architecture, another Scottish firm, was highly commended for the refurbishment of Destiny Church in Glasgow.
The judging panel was made up of AJ editor Isabel Allen, Robin Ellis, of RobinEllis Design and Construction, Piers Gough, Kevin Bonfield, of Boyden, and Rob Wilson, of the RIBA.
Allen said she was particularly pleased with the Patterson Architects' winning scheme. 'This is a project that manages to celebrate the ordinary by being undeniably extraordinary.
'Both the winner and the runner-up demonstrate the vibrancy of Scottish architecture,' she added.
The entire Small Projects initiative - which highlights exceptional projects with a construction budget under £250,000 - is sponsored by RobinEllis Design and Construction.
All the published projects are on show at the RIBA, London, until 18 May.