The short answer
Title and encumbrance, approved layout, land use, access road, setbacks and buildable area — before you pay anything.
Verify the title deed and an encumbrance certificate for the relevant years, confirm the layout is approved by the competent planning authority, and check that the land use permits residential construction.
Then confirm physical access, the setback and floor area rules that apply, and whether the plot is affected by any road-widening or reservation. A title search by a lawyer plus a zoning check by an architect costs little against the risk.
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landtitledue diligence