A Mermhouse is a naturally occurring Structure found in a Marsh biome. Its main purpose is spawning and housing Merms. It has an appearance similar to that of a Pig House, but in a dilapidated state. It cannot be crafted by the player, making it non-renewable. Mermhouses can hold up to four Merms and spawn a Merm every four days if any Merms are killed. Normally, only one Merm per Mermhouse is outside during the day. If that Merm gets into combat or if the house is struck with a Hammer, the rest of the Merms will come out and help it fight. Also, when Dusk comes, all Merms exit their house to wander around in the vicinity.
Deconstructing a Mermhouse with a Hammer yields 1-2 Rocks, 1 Board, and 1 Fish.
In the Shipwrecked DLC, Mermhouses are replaced by Merm Huts in Tidal Marshes. Merm Huts have a more tropical and better maintained appearance compared to Rundown Houses. When destroyed, instead of Fish, they drop Tropical Fish. Otherwise, their behavior is identical.