1. Structure and seismic — invisible, life-defining
Columns, beams, shear walls, foundations. Impossible to redo after build. Upgrading to seismic grade 3 (1.5× code) is worth the +5–10% premium for any home meant to last.
2. Waterproofing — guards the house for 30 years
Roof, balconies, window perimeters. A single leak cascades into finish, structural, and electrical damage. Double-layer roof waterproofing and 5-stage window sill sealing earn their keep.
3. Envelope thermal performance — pays forever
Insulation and high-performance windows. Affects every month's energy bill for the life of the house — JPY 3–5M difference over 30 years. Triple glazing + thermal-break frames + external insulation: just do it.
4. Pipes and electrical capacity — can't be swapped later
Buried in walls and slabs. Once their service life expires, you break walls to replace. Cross-linked polyethylene water lines and a panel with generous spare capacity solve this at design stage.
5. Supervision — independent eyes save quality
Without a design-side supervising architect, you have only the contractor's self-check. Independent supervision at +JPY 2–4M usually returns itself in caught defects alone.
"Visible parts can be updated later; invisible parts can only be done now." Invert this priority and year five will hurt.