But after you account for architect fees, design fees, engineering, and construction costs of $300/PSF+ you are going to end up spending more to build new, almost guaranteed.If you're new to real estate investing and construction, as others have suggested, you should find a house that is already built. 

Second, even if planning is approved, you'll need to hire a structural engineer to get drawings, electricians to reservice wires, a GC to manage the whole operation, etc. 

Find a new architect or tell him you are designing per X budget and after plans are complete if it’s over budget you are not paying for any redesign costs to value engineer then project.

Having a pest inspection done is helpful as well because it notes the areas of dry rot, fungus, termite damage, water damage, etc. and give you actual bids on how much it would cost to repair.(6) is not a major repair, as Greg mentioned.For (2), (3), (5) -- any foundational notes like these I would agree to consult with a structural engineer or expert, as it likely suggests in the home inspection.

So we had all these big gas guzzling cars that got 2-3 miles a gallon in gas and that's when Toyota and Nissan and all of those smaller companies had these 4 cylinder engines.

Once confirmed you could do the addition you would engage an architect and civil engineer to develop plans.3.

Pay the extra money for a structural engineer if you are buying in an area that is notorious for expansive soils and structural issues.

Reverse engineering that means you have $600k in buying power.

STRs can benefit from engineered cost segregation studies but it can be a case by case basis.

The type of adaptive reuse project you are describing might take months or years to plan depending on how busy your engineers, planners, zoning attorneys,  and general contractors are in your market.