Unfortunately I don't have my Jepps to hand for references but there are obstacle clearance requirements that must be adhered to (I think it's 100ft for Cat A a/c).
You are also required to maintain a 2.5% gradient during a missed approach.
This obviously doesn't pose a problem for those flying RPT, however in a twin lightie, chances are the engine out climb performance will not be able to meet these requirements. Therefore, to meet the requirements, you are required to add a saftety margin to your MDA/DA.
This has to be done during the flight planning stage by looking at the single engine performance charts taking into account weight, temerature and pressure etc.