I also have this problem. In v8.5, Mesh>Mirror Geometry>Settings......there is 'Merge with the Original' selection and 'Merge Vertices' radio button.
When I have these selected and click Apply, it...

...and then perform the approach you suggested earlier...right?

Hi Thanks for the response. However sorry to be a pain but your advise is based on not having the door in the first instance & creating it from the 'hole' within the mesh. whereas I'm trying to cut...

Hi All...i'm a complete newbie to the Maya app and am looking to model a car using the app. I'd like some help with creating the door on the car. I imagine this is something easy for most of you, so...