1. Spec changes — the headline killer
"Since we're at it, let's upgrade the floor." "Let's bump the kitchen one level." Owner-driven changes routinely add 10–20%. Mitigation: finish showroom visits during schematic design, then freeze specifications.
2. Ground improvement
The site investigation dictates this — JPY 1–4M is typical when needed. Mitigation: never sign land without a ground survey, and budget for improvement up front.
3. Material inflation
Post-COVID, materials are climbing 5–10% a year. Across an 18-month project, inflation alone can add 5–8%. Mitigation: contract clauses for material-price escalation.
4. Forgotten residual scope
Easy to forget when fixating on the shell:
- Curtains and blinds (JPY 800K–2M)
- Furniture and appliances (JPY 2–5M)
- Moving and utility hookups (JPY 100–300K)
- Exterior planting (JPY 500K–2M)
5. Underestimated soft costs
Title registration, acquisition tax, recurring property tax, insurance. Total JPY 1.5–3M. These rarely sit inside the contractor quote.
"Hold a 10% contingency" is the rule. Plan to spend it; if you don't, that's the upside.