Generally if your trying to place a house a few things will block placement. Items shouldnt hinder plot placement. It is more likely that the corner of the plot is slightly too close to a tree or there is a rock on the ground etc
A general rule with terrain and house placement is - if you can walk through it you can place over it if it blocks your path you cant (that is a general rule there may be a few exceptions).
You could try using an axe on the water trough (if its floating you may need to move around the object to find the right spot to allow you to do this).
Items left after an idoc will generally get pushed to the side (under house sign) when you place the plot so as said before should not stop you placing. If you reckon it is water trough and cant remove it try paging a gm to see f they can get rid of it for you.
Also, and this is just incase - Make sure you are not getting the housing can not be placed in this area message
