The only way to make this look intentionally designed is to close in the opening and stucco it to look like the rest of the home. You can place a window in it that matches the size and scale of the rest of the home. You will also need to eliminate the driveway that runs straight into the opening, and reroute it to the new garage.

If you can come up with the money to get some of these convertible units, the space can be even more versatile. I got the Expand coffee table that converts from a short table that fits easily in front of the sofa, to a full dining height table twice the size. I can use my sofa as "booth" seating and pull up four chairs and seat six! Something as simple as the lift and storage bed can give your sister so much more usability of her space. So can a console that pulls out to be a dining table, or a table that folds into the wall. I bet if you did a search on YouTube you could find some place to buy the plans for making some of these things. Expand Furniture in Vancouver, BC, CA I know that the tiny homes shows on TV use the bed lift storage systems, so they must be for sale. Wayfair sells a ton of storage coffee tables that convert to bring a writing or dining surface up to the person seated on a chair or couch. It is not all expensive, either! There is often a space under the surface that can store things, too. Don't forget to use the wall cavities between studs for storage whenever you can. I have seen nice toilet brush and plunger cabinets next to toilets, recessed niches for display (with a light at top), and spice storage for kitchens. WG Wood Products will give your husband lots of ideas for using wall storage.

Alternately: What you do depends upon what will be needed within the shop -- amount of natural light and/or blank wall space for building storage/countertop/workspace along a wall, whether or not the size of the garage should be enlarged by extending the left end of the house. Is the front of the house facing north or south or east or wet -- will sunlight go directly into the windows and, if so, what time of day? How much natural light will shine in from other sides and is a lot or little natural light desirable? Do you want fixed windows or do the windows need to open? Is the back wall of the garage brick or the same siding as the end? Would it be desirable to move the two front garage doors to the back of the house? -- or just move ONE of the garage doors to the back wall so it is directly across from the other garage door you do leave on the front -- a drive thru garage or just a great breezeway to enable dust/fumes from the shop/garage to exit when both doors are open?

I can't tell you how much I dislike garages that are in my face in front of an entry to make the front of the house all garage as the first thing you see. But that said I do not like bedrooms above garages either. A garage is not living space so you will need to dempolish the garage and then build an addition just like any other addition. Can you do what you want where you live there are often rules about sq footage of living space in proportion o the lot. As for cost that can only be decided after getting all the info you need and then quotes from contractors in your area. IMO you also need an architect since IMO additions are best when they look like they have always been there