What separates a plot that trades at a discount from one worth building against.
A plot trading at a discount usually has a reason — access, boundaries, zoning, or a constraint that has kept better-capitalised buyers away. The work is separating the fixable problems from the permanent ones.
When the discount comes from something you can resolve — a boundary line, a servitude, a missing road frontage — the uplift on fixing it is often the whole return. When it comes from something structural, no price is low enough.