Acheter vs louer (Vestafolio)

Required workflow

For a request within this simulator's scope:

1. Reuse answers already supplied. Ask the missing questions below before giving a numerical result or a personalized recommendation. Example values and schema defaults are not the user's answers. City, property type, condition and surface only help suggest missing assumptions; they are not required when the user supplies the corresponding rates, notary fees and annual property tax. With complete calculation inputs, proceed to the API.
2. Before sending inputs, explain that this calculation uses Vestafolio's external API at https://www.vestafolio.com, summarize the fields and values to be sent, and obtain the user's agreement. Existing explicit approval for this destination and data scope is sufficient; do not ask again for calls it already covers. Installing or selecting the skill alone is not approval to transmit personal data. If the user declines, do not POST; offer the simulator link for manual use without prefilled inputs.
3. Once inputs and approval are known, actually call a tool: fetch the schema, then POST only the relevant simulator fields to the endpoint below. Omit unused optional data, names, addresses, account identifiers, credentials, files, and conversation history. No authentication is needed. Use an available HTTP tool, a terminal with curl, or Python code execution (execute_code in OpenWebUI). Python can use urllib.request; in browser-based Pyodide use await pyfetch(...) from pyodide.http. A Python environment does not need a shell to call the API.
4. Check HTTP success and the POST envelope: ok must be true; read the calculation from result. Ground the answer in that output, state relevant assumptions and limits. Offer the simulator link when useful for checking assumptions or exploring alternatives; respect requests for no links.

Writing a code block is not execution. Do not substitute mental arithmetic, remembered tax rules, or the worked example for a tool result. If execution or network access is unavailable, or the API fails, say the calculation could not be completed and provide the simulator link; do not invent its result or recommendation. A schema GET alone is not a completed simulation.

Response language

Reply in French whenever the user speaks or writes in French.

Compares the final net wealth of buying a primary residence with a mortgage versus staying a tenant and investing the savings, simulated month by month over a chosen horizon: loan amortization, property appreciation, rent increases, taxe foncière (indexed at 3 %/year) and capitalization of invested savings. The model below is the one coded in the simulator; use it to explain, and the API to compute.

When to use

• "Should I buy or keep renting?" / "Acheter ou louer ma résidence principale ?"
• "After how many years does buying beat renting?" (break-even questions)
• Testing sensitivity to property appreciation, investment returns, rent growth or horizon

When NOT to use

• Buy-to-let / rental-investment profitability — use vestafolio-rentabilite-locative
• Sizing the loan or its payment alone — use vestafolio-capacite-emprunt or vestafolio-credit-immobilier
• Choosing where to invest the savings themselves — use vestafolio-pea-vs-cto

Questions to ask before calling the API

The simulator asks these inputs, in this order, and starts with the savings on purpose: they bound the loan share and the works budget. Ask only for missing calculation inputs in French; reuse explicit values without asking for confirmation. Collect descriptive details only when needed to suggest a missing assumption, then let the user confirm that assumption.

« Hypothèses de marché »

1. « Épargne disponible actuellement » (gate) → availableSavings. It funds the apport, the notary fees and the works in the buy scenario and is fully invested in the rent scenario.
2. « Ville » — not sent to the API; the simulator uses it to suggest the appreciation, rent-increase and taxe foncière assumptions below. Ask it when the user has no own assumptions.
3. « Appréciation immobilière » → propertyAppreciation (%/an). Simulator suggestions by city for a horizon ≤ 5 / ≤ 10 / > 10 years: Paris 1,5 / 2,5 / 3 ; Lyon 2 / 2,5 / 2,8 ; Marseille 2,5 / 2,8 / 2,5 ; Bordeaux 1,8 / 2,2 / 2,5 ; Toulouse 2,2 / 2,5 / 2,5 ; Nice 2 / 2,3 / 2,5 ; Nantes 2 / 2,5 / 2,8 ; Strasbourg 1,8 / 2,2 / 2,5 ; Montpellier 2,2 / 2,5 / 2,5 ; Lille 2 / 2,3 / 2,5 ; Rennes 2,2 / 2,5 / 2,8 ; Grenoble 1,5 / 2 / 2,2 ; Rouen 1,5 / 2 / 2,2 ; Toulon 2 / 2,3 / 2,5 ; Angers 2 / 2,3 / 2,5 ; other city 1,5 / 2 / 2,2. Present the suggestion and let the user confirm.
4. « Rendement placements » → investmentReturn (%/an) and investmentReturnBuy: the website uses one rate for both scenarios, so send the same value in both fields unless the user explicitly wants the owner's leftover savings invested differently.
5. « Horizon de projection » → horizonYears (1 to 30 on the site).

« Scénario Achat »

6. « Type de bien » (Appartement / Maison), « État » (Ancien / Neuf) and « Surface » — not sent to the API. Skip these questions when notaryFeePercent and taxeFonciereAnnual are already supplied. Otherwise, they drive the notary-fee suggestion (7,5 % ancien, 2,5 % neuf) and the taxe foncière estimate (per m² per year, appartement / maison: Paris 15-35 / 20-45 ; Lyon 12-25 / 15-30 ; Marseille 10-22 / 12-28 ; Bordeaux 14-28 / 16-32 ; Toulouse 11-24 / 14-28 ; Nice 13-30 / 18-40 ; Nantes 12-24 / 14-28 ; Strasbourg 10-22 / 12-26 ; Montpellier 11-24 / 14-28 ; Lille 13-28 / 16-32 ; Rennes 11-23 / 13-27 ; Grenoble 10-22 / 12-26 ; Rouen 12-25 / 14-28 ; Toulon 11-24 / 14-30 ; Angers 10-20 / 12-24 ; other 8-20 / 10-25).
7. « Prix du bien » → purchasePrice.
8. « Part empruntée » → loanAmount = price × share. Simulator rule: « Un apport minimum équivalent à 10% de la valeur du bien est exigé. Votre apport + frais de notaire ne peuvent excéder l'épargne disponible. » So the website limits the loan share to 90 % of the price. Its cash budget must cover apport + notary fees + works. A savings constraint limits the cash contribution; it is not a maximum borrowing capacity. These website controls are distinct from the API validation described below.
9. « Taux crédit » → loanRate and « Durée emprunt » → loanYears.
10. « Frais de notaire » → notaryFeePercent (percent of the price; 7,5 for an ancien, 2,5 for a neuf — chain vestafolio-frais-notaire for an exact figure).
11. « Taxe foncière annuelle » → taxeFonciereAnnual (indexed at 3 %/year by the simulation).
12. « Travaux » → travaux, paid cash. Must not exceed savings − apport − notary fees (« Maximum autorisé »); above it the website freezes the simulation and the API returns a validation error with the ceiling.

« Scénario Location »

13. « Loyer mensuel » → monthlyRent (equivalent housing).
14. « Augmentation annuelle du loyer » → rentIncreaseRate (%/an; simulator suggestion by city: Paris 2,5 ; Lyon 2,3 ; Marseille 2 ; Bordeaux 2,2 ; Toulouse 2,1 ; Nice 2,4 ; Nantes 2,3 ; Strasbourg 2 ; Montpellier 2,2 ; Lille 2 ; Rennes 2,3 ; Grenoble 1,8 ; Rouen 1,8 ; Toulon 2 ; Angers 2 ; other 1,8).

Model as coded in the simulator

• Buy scenario: the apport (price − loan), the notary fees and the works are taken from the savings once; whatever remains is invested at investmentReturnBuy. The owner pays the mortgage (constant annuity, no insurance) and the taxe foncière indexed at 3 %/year from year 1.
• Rent scenario: the full savings are invested at investmentReturn; rent grows at rentIncreaseRate from year 2.
• Effort d'épargne: each year, whichever side has the lower housing outflow (mensualités + taxe foncière for the owner, rent for the tenant) invests the difference. Savings earn interest from year 2; an effort saved in year N earns interest from N+1.
• Property value = price in year 1, then appreciates each year. Intermediate years value the stake as property value × repaid share of the loan; at the horizon the stake is the sale proceeds (property value − outstanding principal).
• Net wealth (buy) = stake − cumulated taxe foncière − cumulated loan interest + investment portfolio. Net wealth (rent) = portfolio − cumulated rents.
• recommendation = buy when the difference is strictly positive, else rent; breakEvenYear = first year where the buy wealth reaches the rent wealth (null if never). The website words it « L'achat génère X € de plus » / « La location génère X € de plus ».

How to call the API

Always fetch the canonical input schema first (fields, bounds, defaults):

GET https://www.vestafolio.com/api/tools/v1/achat-vs-location

Then POST the user's parameters (all amounts in euros, rates in percent):

curl --fail-with-body --silent --show-error --max-time 30 -X POST https://www.vestafolio.com/api/tools/v1/achat-vs-location \
  -H 'Content-Type: application/json' \
  -d '{
    "availableSavings": 80000,
    "propertyAppreciation": 3,
    "investmentReturn": 6,
    "investmentReturnBuy": 6,
    "horizonYears": 20,
    "purchasePrice": 300000,
    "loanAmount": 255000,
    "loanRate": 3.5,
    "loanYears": 20,
    "notaryFeePercent": 7.5,
    "taxeFonciereAnnual": 1500,
    "travaux": 0,
    "monthlyRent": 1200,
    "rentIncreaseRate": 2.5
  }'

Note the input constraints: loanAmount must not exceed purchasePrice, and travaux must fit within savings after apport and notary fees (the response's maxTravauxBudget gives the ceiling). Unknown fields are rejected (strict schema) — if you get a validation_error, re-read the schema from the GET endpoint rather than guessing field names. For invalid user values, explain which supplied field was rejected and ask the user to correct it. Do not invent a financing ceiling or substitute a new loan amount. For example, loanAmount > purchasePrice calls for clarification of those two amounts, not an unsolicited borrowing-capacity calculation. Only describe a numerical limit if it was returned by the tool, and keep its field and meaning intact.

Interpreting the output

• buyWealth / rentWealth / difference — final net wealth of each scenario and their gap (positive difference = buying wins)
• recommendation — "buy" or "rent" at the chosen horizon
• breakEvenYear — highlight it when the user's horizon is close to it (moving before break-even favours renting)
• monthlyPayment — the mortgage payment (hors assurance) driving the monthly-effort comparison
• buyDetails / rentDetails — decomposition (property value, remaining loan, cumulated interest and taxe foncière, portfolios, additionalSavingsInvested); use it to explain WHY one side wins, like the site's « Détails Achat » / « Détails Location » blocks
• yearlyData / cashflowData — year-by-year wealth curves and outflows, for "what if I sell after N years" follow-ups
• maxTravauxBudget — the works ceiling for the given savings

Caveats

• Sensitive to assumptions: the recommendation can flip with 1-2 points of propertyAppreciation or investmentReturn — present a sensitivity check, not a verdict.
• Not modeled: assurance emprunteur, copropriété charges and maintenance, selling costs, capital-gains rules, tax on investment returns.
• Estimates, not financial advice — say so.
• Interactive simulator, if useful to the user: https://www.vestafolio.com/simulateurs/achat-vs-location