Re: Making a Single Entry Door into a Double
Every opening has a header to support the structure around the opening, however that header is only as wide as the existing opening. To widen your doorway, you will need to open up the wall and install a new, longer header that will accommodate the new door width.
If your entry is just a bit wider than what the new door will be, then I'd suggest removing the lath and plaster from wall to wall and floor to ceiling to give you access to the framing and any electrical that will need to be relocated. Once the new door is in you can install drywall instead of lath/plaster. You shim the studs so that the finished surface of the drywall will be at the same height as the plaster.
As for the exterior, it is usually easier to remove "fake" siding and reinstall it after the work is finished rather than trying to cut/modify it in place. The reason being is that you're going to have to deal with whatever the original wall surface was too. I'd recommend consulting with a fake siding installer for assistance.
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