Contributory Values for site improvements (including houses, garages, exterior amenties - driveways, patios, decks, landscaping, fences etc.) are local market specific i.e. and/or neighborhood specific and often dwelling age, dwelling style, actual age, effective age, quality, above and below grade utility (+/- finish), view, lot size and utility, and additional site improvements determine market demand for improvements.
IOW - extracting contributory values for all the elements above. What OTHER buyers of OTHER competitive properties actually PAID $$ ( note: cost is not a comparative element on resale properties) determines the answer to your question AS OF a specific DATE. An owner may get a ROI on improvements equal to, above, below or NO return on RE-sale.
One tool to use is comparative analysis of ACTUAL closed, and recent Competitive Properties. Remember Listing Agents "cma" may be helpful or misleading IF the preparer does not remain entirely Neutral and Objective. CMAs to determine Listing Prices done to OBTAIN a new listing is often jaundiced at the get go.
Suggestion: as Appraisal Values are OPINIONS which should illustrate Contributory Values - have 3 appraisals done by 3 different Appraisers on a "what if I added xxxxx<< new improvement or improvements" basis.
Consumer tastes change, markets change, Agents who perform "pre-listing" CMAS are seeking the Listing to Sell and have commissions riding on a sale.
Independent and Objective Appraisers could care less what the $$$ value is - NO Escalating Commission riding on the Opinion. "it is what the competitive market says it is".
Finally, the last sentence is of utmost import. Simply buyers of the most similar, recent, and proximate to yours determine "market value" for competitive properties inclusive of location, (geo and often school district), view, lot size, lot utility and topography, dwelling style, age, design, and all the other improvements which may currently be worth the same as what they cost, less than what they cost, zero contributory value (over-improvement), OR add MORE than cost.
IT DEPENDS.
In my local markets savy owners wisely retain the services of an experienced LOCAL Appraiser to answer "the question" BEFORE they make a $$ investment.
IOW - what would current market value for my property be IF ..................I added, etc. etc. etc.