Core answer: Safe monthly mortgage payment = household take-home income × 40%. With ¥20k/month take-home, the payment ceiling is ¥8,000 — at 30 years / 3.0% that supports a loan of ~¥1.9M, or a home of ~¥2.7M with 30% down. Keep another 5%–8% of the price aside for taxes and renovation.
Four red lines for affordability
- Payment line: mortgage payment ≤ 40% of household take-home pay (prudent line); 50% is the bank's approval ceiling — buying at the ceiling is tightrope walking
- Down-payment line: beyond the down payment, keep a 12-month-payment emergency fund
- Total-debt line: all debt payments (car loans, consumer loans included) ≤ 50% of income
- Stability line: single-income households in volatile industries should shave another 5–10 points off the 40%
Payment → loan quick table
30-year equal-payment loan at 3.0% (¥42.16/month per ¥10k borrowed):
| Monthly payment | Loan supported | Price with 30% down |
|---|---|---|
| 4,000 | ¥950k | ¥1.36M |
| 6,000 | ¥1.42M | ¥2.03M |
| 8,000 | ¥1.90M | ¥2.71M |
| 10,000 | ¥2.37M | ¥3.39M |
| 15,000 | ¥3.56M | ¥5.08M |
Every 0.5-point rate rise cuts the supportable loan by ~5% at the same payment.
Income → home price table
At 40% of take-home, 30% down, 30yr/3.0%:
| Monthly take-home | Safe payment | Loan | Home price | Extra needed (tax + renovation) |
|---|---|---|---|---|
| ¥10k | 4,000 | ¥950k | ¥1.36M | ¥100–180k |
| ¥15k | 6,000 | ¥1.42M | ¥2.03M | ¥140–260k |
| ¥20k | 8,000 | ¥1.90M | ¥2.71M | ¥190–350k |
| ¥30k | 12,000 | ¥2.85M | ¥4.07M | ¥270–500k |
| ¥50k | 20,000 | ¥4.74M | ¥6.77M | ¥450–800k |
Three households, worked through
Case 1 (newlyweds, combined take-home ¥24k): safe payment ¥9,600 → loan ¥2.28M → price ¥3.25M. Savings ¥1.1M; after the ¥980k down payment only ¥120k remains — short of the 12-month emergency fund (¥115k) plus taxes and renovation (¥200k+). Verdict: drop to a ¥2.8M target or save another year.
Case 2 (single, take-home ¥12k): safe payment ¥4,800 → loan ¥1.14M → price ¥1.63M; single-income risk argues for a 10% discount — shop at ~¥1.45M.
Case 3 (two kids, take-home ¥40k, car loan ¥3,000/month): total-debt check (12,000+3,000)/40,000 = 37.5% < 50% ✓; but education costs are rigid and rising, so budget on a ¥10,000 payment — a ~¥3.4M home.
Hidden costs beyond the down payment
| Item | Scale |
|---|---|
| Deed tax | 1%–3% (1% for first home under 90 m²) |
| Agency fee (resale) | 1%–3% |
| Maintenance fund | ~¥100–200/m² |
| Renovation + appliances | from ¥1,000–1,500/m² |
| Property management | ¥2–4/m²/month, forever |
| Heating (northern cities) | ¥20–30/m²/year |
Total ≈ 8%–15% of the price, budgeted separately from the down payment.
Stress tests before signing
- Job-loss test: if one income stops for 6 months, do savings cover payments + living costs? (12-month emergency fund = pass)
- Rate test: a 1-point rate rise adds ~6% to the payment — still affordable?
- Lifestyle test: after the payment, can the remainder fund "an undegraded basic life + at least 10% monthly savings"?
Common mistakes and myths
- "Buy whatever the bank approves" — banks lend up to 50% of income; at that line, any income wobble becomes a default crisis. Approval ceiling ≠ affordability.
- "Just get on the ladder first" — emptying six wallets for the down payment and funding taxes/renovation with credit cards is double high-interest leverage with no exit if prices stall.
- "Housing-fund balance isn't real money" — housing-fund can be withdrawn toward the down payment and offset monthly payments; treat it as hidden income in your math.
- "Renting is pure loss" — in cities with rent-to-price ratios below 2%/year, renting costs far less than interest + opportunity cost of owning; "loss" is a calculation, not a feeling.
Use the [Home Affordability Calculator](/c/finance/house-affordability) to convert income into a price ceiling, and the [Mortgage Calculator](/c/finance/mortgage) for exact payments.