A quick Google search will provide a plethora of Royal Icing recipes for a gingerbread house. They are all pretty much the same with small variations in amount of water, which will always be a variable from day to day anyway.
I start with this Royal Icing recipe and tweak the amounts of water needed based on humidity and what I am need it for. Sometimes I want the icing a little thicker and dryer, and other times softer and wetter, and sometimes I'm flowing with the same icing!
GBD Royal Icing
6 Tbsp (56 g) Wilton Meringue Powder (results may differ with other brands)
8 cups (2 lbs or 906 g) powdered / confectioners / 10X Sugar
3/4 cups (180 g) warm water
2 tsp Vanilla, or other flavoring if desired
Store leftover Royal Icing, tightly covered, in the refrigerator for weeks.
This Royal Icing recipe makes about 7 cups of glue frosting.
Note: 6 egg whites may be substituted for 6 tablespoons of meringue powder. Omit the water if using real egg whites.
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Great recipe! Dries well and it holds things together!