In the first instance, the presence of do makes for a strong suggestion that contradicts a common assumption. Please, do also visit this house would be in addition to doing something else that is not house visiting, such as filing application paperwork. I can imagine it in this context:
I would very much like to entertain your request for full membership in our fraternity. You must however fulfil certain requirements. First, file your papers in the main office. Then, knit fifteen colorful scarves. Leave these in the trees around the dean's house. Please, do also visit this house in the morning so that we might assess your courage.
In this case, you wouldn't expect that someone pranking the dean's house would then visit the house the following morning.
In the second case, it is simply doing something in addition to house visiting.
Come by the hospital around noon to pick up the roses. Choose your shoes from the shop around the corner. Please also visit this house on your way home.
Lastly, as Luke says, if you mean to express that there is an additional house to visit, you would normally say please visit this house also, rather than either of the choices you gave.