Compact buildings depreciate very differently depending on three things: the structure, the paperwork and whether the buyer can put it anywhere.
The structure sets the floor
A steel frame with a correctly executed envelope is a fifty-year building and is valued as one. A light timber leisure unit is designed for seasonal use and is valued on a depreciation curve closer to a caravan.
This is the single largest factor and it is decided at purchase. Nothing you do afterwards moves a building from one category to the other.
Paperwork is worth real money
A documented build — frame photographs, insulation build-up, electrical certificate, flue record, maintenance log — lets a buyer verify what they are buying. Without it, they are pricing in a risk, and the discount is larger than the cost of keeping the file.
Start the file at handover. Reconstructing it three years later is impossible for exactly the parts that matter.
Siteability is what the buyer is actually buying
A house that fits standard road transport, needs no exceptional permit to move and has a documented foundation interface can go anywhere. One that is 3.2 metres wide is worth less because moving it is a project.
The 2.55 metre width is not just a delivery convenience. It is what keeps the second buyer's options open.
What ages badly
Highly specific interiors, unusual colours, bespoke fittings that cannot be replaced, and anything that dates hard. In a small house every one of those is a large proportion of what the buyer sees.
Keep the fixed elements neutral and put the personality into the things that move. That is not a compromise; it is what makes the house work for the next occupant too.
The market for used compact houses
It exists, it is thin, and it moves on trust. Photographs from the build phase, a moisture reading and a look at the chassis underside are what an informed buyer checks — which means those are what an informed seller prepares.
Price against comparable new plus the cost of the site work already done. A house already sited, connected and permitted is worth more than the same house on a lorry.