Preheat oven to 300°. Cut out patterns for cottage walls and roof.
In a large bowl, whisk together flour, salt, baking powder and spices. In another bowl, whisk together egg, molasses, oil and brown sugar; gradually beat into flour mixture, mixing well. Press evenly into a well-greased 17x12x1-in. pan. Bake until golden brown and top springs back when lightly touched 25-30 minutes.
Immediately top gingerbread with patterns; cut with a knife. Cool completely on a wire rack (gingerbread will harden as it cools).
For icing, beat confectioners' sugar, water, meringue powder and cream of tartar on low speed until blended; beat on high until stiff peaks form, 4-5 minutes. (Keep unused icing covered at all times with a damp paper cloth; if necessary, beat again on high speed to restore texture.) Transfer icing to a pastry bag or food-safe plastic bag fitted with a pastry tip.
Pipe icing along base and edges of front wall and one side wall. Position at right angles to each other and place on a cardboard base; prop up with small jars. Repeat with back wall and a second side wall; let dry. Remove jars.
For roof, pipe icing along peaked edges of front and back walls; position roof pieces. Let dry. Decorate as desired with remaining icing and coarse sugar.
Meringue powder is available from Wilton Industries. Call 800-794-5866 or visit wilton.com.