What a fascinating concept. The Cellars deed is considered a house add on and will grant you 1 additional floor below your current house foundation. Acquiring these cellar deeds is available through our donation shop only at this time. When using the deed, you will be prompted to choose the type of basement floor / wall design. The top right green bubble button can be used to change to the next page of options.
Once you find a floor design you like, target a location in your house to create this grandfather clock.
Right now this entrance item is limited to the grandfather clock. Double clicking on it will teleport you to your new cellar. Cellar size is based on the foot print of your foundation. Do note that once a floor design is changed, it can not be adjusted. The only workaround at this time is to page staff or to use the house add on deed key to restore it.