Process flour, spices and butter until mixture is crumbly (you may have to process in two batches). Add sugar, treacle and enough egg for mixture to just combine. Turn dough onto floured surface; knead until smooth. Cover; refrigerate 1 hour.
Cut paper patterns for gingerbread house: two 12cm x 19cm rectangles for the roof, two 10.5cm x 16cm rectangles for the side walls, and two 16cm x 18cm rectangles for the front and back walls. Trim front and back walls to form two 11cm high gables.
Preheat oven to moderate (180°C/160°C fan-forced). Roll dough between sheets of baking paper until 5mm thick. Peel away top paper and use patterns to cut shapes from dough. Pull away excess dough. Slide baking paper with shapes on to oven trays. Bake, uncovered for about 12 minutes or until shapes are just firm (they become crisp after they cool). Re-roll dough scraps into one 5mm-thick piece. Cut out tree and chimney.
While shapes are still warm and soft, use tip of sharp knife to cut out small windows from the side walls of the house, then cut the door out of the front wall. Reserve the cut-out door piece. Trim shapes to straighten sides. Transfer all shapes to wire rack to cool.
For the royal icing, beat egg whites in small bowl with electric mixer until frothy; gradually beat in sifted icing sugar. Cover icing with damp cloth while not using.
Cover board with foil or silver paper.
Secure two crossed skewers to back of each roof piece with icing. Allow to dry.
Assemble house, securing roof and walls together with icing. If possible, stand house several hours or overnight, supporting sides with four cans, so that it is thoroughly dry before decorating. Decorate board around house with remaining icing to resemble fallen snow. Secure door to house with icing. Decorate house with lollies, secure with icing. Secure trees to board and chimney to roof with icing.
Dust house with a little sifted icing sugar, if desired