Developing an Add-in (Plug in) for Revit Architecture 2012

I need to develop an add-in for Autodesk Revit Architecture 2012. As you may know Revit is an object oriented modelling software and all of the components used in a Revit model has some attributes (such as height, width, length, start date, finish date, etc.). What I want to do is to develop an add in which makes query (for example select all the components with the start date of X) and automatically generate a bounding box (a box which contains the whole object) around it. I would like to be able to add different margins for different sides of the bounding box before generating it. I also attached a sample C# code which gives the size of the bounding box by selecting a component and may be used as a prototype. Please contact me if you need any further information.