In a large mixing bowl, combine flour, ginger, cinnamon, and baking soda. In a separate bowl, beat butter and brown sugar until smooth. Mix in molasses and egg. Gradually add dry ingredients to form a firm dough.
Roll out dough on parchment paper to about 1/4-inch thickness. Use templates or cutters to cut 2 walls, 2 roof panels, and 2 gable pieces.
Preheat oven to 350°F (175°C). Transfer gingerbread pieces to a baking sheet lined with parchment paper. Bake for 12–15 minutes. Let cool completely.
In a mixing bowl, combine powdered sugar, meringue powder, and water. Beat until stiff peaks form. Adjust consistency as needed.
Use royal icing to glue the walls to a flat surface. Hold or support until stable. Add roof pieces and allow the structure to set for 15–20 minutes before decorating.
Use piping bags or sandwich bags to add icing details. Stick candy corn, licorice strips, marshmallow ghosts, and gummy spiders onto the house. Color icing with gel food coloring for spooky effects. Let dry at least 1 hour.