Wurt's W.I.P. kit does not drop any built-in materials when hammered.

The Merm Tapestry W.I.P. kit Wurt can craft and place down, does not return any of the kelp, pig skin, or beefalo wool you built into it when hammering, meaning you lose out on everything you built into if if you decide you'd like to change the building area to another place. This is a missing feature, since currently the W.I.P. kit is the only hammerable construction project of it's kind, which would explain why there is no functionality for construction projects dropping their materials when hammered.