Beat sugar and butter in a mixer until pale and fluffy.
Add treacle, syrup, spices, bicarb, and flour, then beat into a stiff dough using water as needed.
Knead dough on a floured surface for a few minutes until smooth and rest in a bowl.
Cut card templates for gables (20cm x 14cm), back (30cm x 14.5cm), and front with arches.
Preheat oven to 180°C.
Roll dough to 3mm thickness, cut shapes using templates, and bake for 15-18 minutes.
Whisk icing sugar and egg whites for 7-10 minutes until thick and smooth.
Pipe icing along edges and assemble walls on a stiff board using tins for support if needed.
Attach gables to the back and front walls at intervals.
Pipe icing on roof edges and press gently onto the gables to cover the garage.
Allow to set for 30 minutes before decorating with sweets and icing.