Preheat oven to 180°C. Line 2 large baking trays with baking paper. Divide gingerbread dough into 6 portions.
To make the side walls, roll out 1 gingerbread portion on a lightly floured surface to a 4mm-thick disc. Cut into a 12cm x 13cm rectangle. Place on 1 lined tray. Repeat with another gingerbread portion. Use a 3cm heart cutter to cut 2 hearts from each wall for windows. Place hearts on the lined tray.
To make the roof, roll out 1 of the remaining gingerbread portions on a lightly floured surface to a 4mm-thick disc. Cut into a 13cm x 15cm rectangle. Place on the tray. Repeat with another gingerbread portion.
To make the front and back walls, roll out the remaining 2 portions on a lightly floured surface to a 4mm-thick disc. Cut each into a 12cm x 11cm square with a 9cm-long triangle on the end to form a pitched roof. Place on lined tray. Use heart cutter to cut 1 heart from each wall for the windows. Use a small sharp knife to cut a door from the front wall. Place the hearts and door on the lined tray.
Bake gingerbread for 15-20 mins or until golden brown and cooked through. Set aside on trays to cool completely.
To make royal icing, whisk egg white and lemon juice in a bowl. Gradually add icing sugar, stirring after each addition, until smooth, adding more icing sugar if necessary to make a thick paste.
To decorate the roof, spread the roof pieces evenly with a thin layer of icing. Place 100’s & 1000’s on a plate. Dip roof pieces, icing-side down, in 100’s & 1000’s to evenly coat. Set aside to set.
To decorate the walls, spread each wall with icing. Arrange sour straps over the walls, using the heart cutter to cut out the windows. Place a little icing in a piping bag fitted with a 2mm plain nozzle. Pipe icing around all the windows and door. Set aside to set.
Spread heart shapes and door with icing and sprinkle with 100’s & 1000’s.
To assemble the house, spread a little icing over the side edges of the front, back and side walls. Join together, using a straight-sided glass to support the sides. Set aside for 1 hour to set.
Spread a little icing over the top edges of the walls and attach the roof pieces. Place a glass under the bottom edge of the roof to support. Set aside for 1 hour or until completely set.
To finish, use icing to attach door to the front. Pipe over the roof to make tiles. Use icing to attach M&M’s to the top. Pipe snow at the top of the front and back walls. Use kitchen scissors to cut remaining sour strap into triangles. Use icing to attach to the front wall to make bunting. Set aside until set.