My home is on a good plot and is modest in comparison to the Neighbor's, so I should recoup most (75%) expenses if I do ever decide to sell.
I don't know what exact area you are in but I find it very, very difficult to believe that you are going to see that type of return on investment with a detached garage with an apartment over it. Unless you live in some strange municipality in which it is usual and customary to have a detached garage with an apartment over it. There is so much complexity that goes into how a property is appraised and listed by an agent for sale. You can't claim the square footage of the apartment when listing the home's square footage because it is not an extension of the home. Further, a lot of banks will be leery of financing somebody for a mortgage at times if the property has one of these spaces. They don't want the buyer doing the math of figuring they can buy the house and make ends meet by renting out the apartment.
I once looked at a home that had a great little Mother-in-law efficiency that joined the house via the kitchen. Had its own little kitchenette, bathroom, exterior entrance, etc. I found it odd that the space was not separated from the main house by any door. Upon further inspection it appeared as though there was fresh paint and new trim in that space. The realtor came clean and said that it was required by the flipper's bank to remove it so as not to throw red flags on an appraisal by a prospective buyer's bank.
I think you should take the time to talk to a realtor about what you can expect to recoup on this project. Call any agent with a sign up in your 'hood and explain what you are trying to do. They will most likely give it to you straight. I think you are going to be looking at what most guys recoup on detached garages: jack squat, especially if the home already has an attached garage in keeping with what is normal for the area.
Not trying to be the Debbie Downer but when I hear someone throw out a number like 75% ROI on a detached shop, it throws red flags. Do your due diligence in talking to that architect and a realtor so you can go into this with open eyes and a clear head. Both professionals have the expertise to save you a lot of heartburn.