I would like to approach this question from a couple of different angles. The first angle will be from a lenders perspective, since you question is about value.
If you have a doublewide mobile home on land, that has it’s title removed and is secured permanently- and is in all respects an FHA approved mobile home, then breaking the skin of the mobile will negate that fact. Meaning no one (including you) will ever be able to get an FHA loan on your mobile home.
The reason this is important is that as FHA loans do not cost more (as long as it meets the guidelines above) and they do not require more down payment or anything extra special. This will increase the number of people who can qualify to buy the home and thereby increase the value.
If you live in a mobile home that does not meet FHA guidelines, then an addition might ad some value but will probably not be equal to the cost of the improvements since you would be limiting the market of buyers, and thus the determined value would be lower.
As a homeowner it may be worth a ton to you, since how you enjoy the property can affect your life quality.
Market value will be determined by comparable or “like” properties also so a one of a kind property or hybrid property will always be difficult to get the value out of.
However, if you have owned the land for more than 12 months you may be eligible for a refinance construction loan. Included in the loan could be the removal of the mobile home and you could build a stick built home on the property. All of this of course would be subject to your lot size, zone codes and permits etc.
Hope that helps!