New vs old property: simulate both scenarios before you decide

New vs old property: why simulation matters more than rules of thumb

Choosing between a brand-new apartment and an older place that needs renovation is not just about the listing price. To really decide whether to buy or rent, and to compare new vs old building, you need to quantify two key simulator parameters: montant_fn (property tax amount) and montant_travaux (renovation and works budget).

Over 20–25 years, these two items can add up to tens of thousands of euros and completely change the outcome of your buy or rent calculation. That’s why a proper new vs old building simulation is far more reliable than generic advice like “new is always better” or “old has more potential”.

Core assumptions before comparing new vs old

Before zooming in on property tax and renovation, any serious buy or rent analysis should include a consistent set of assumptions:

• Mortgage rate: around 3.6% currently for 20–25 years.
• Notary fees: typically 2–3% for new builds, 7–8% for existing properties.
• Agency fees: often 3–5% of the purchase price.
• Borrower insurance rate: about 0.25–0.45% of the loan amount per year.
• Annual rent increase: indexed to rent indices, often 2–4% depending on inflation.
• Investment rate: return on savings (cash, bonds, ETFs, etc.).
• Annual inflation: erodes the real value of both rent and mortgage payments.
• Prepayment penalties: usually capped at 3% of remaining principal or 6 months of interest.

Within this framework, two parameters are decisive when comparing new vs old: montant_fn (property tax) and montant_travaux (works). A robust buy or rent simulator must let you input different values for these in each scenario.

Montant_fn: property tax can diverge massively between new and old

1. What the montant_fn parameter represents

In a buy or rent calculator, the parameter montant_fn corresponds to the annual property tax. It varies a lot depending on:

• The municipality and local tax rates.
• The assessed rental value (size, quality, location of the dwelling).
• The type of building: new vs old.

On top of the starting level, you must factor in annual property tax increases. In many cities, property tax has risen faster than general inflation in recent years. A 3–5% yearly increase over 20 years becomes a major cost in any buy or rent simulation.

2. Temporary advantage for new builds: partial exemptions

Many local authorities grant a partial or full exemption from property tax for the first 2 years (sometimes more) on new constructions. In practice:

• Years 1–2: property tax is reduced or even zero.
• Year 3 onward: you move to the normal tax regime, with annual revaluation.

Simple example:

• New flat: montant_fn = €0 for the first 2 years, then €900 from year 3.
• Old flat: montant_fn = €1,200 from year 1, increased by 3% per year.

Over 10 years, the cumulative difference can easily exceed €5,000–7,000, which can tip the balance in your new vs old building simulation.

3. Older properties: higher property tax, but not always

Market data often shows that:

• Older units in city centres tend to carry higher property tax (e.g. €1,000–2,000+ per year in large cities).
• New developments on the outskirts may start lower, but their tax level can catch up quickly as the area develops.

In your buy or rent simulator, it is crucial to:

• Enter a realistic montant_fn for each scenario (new vs old).
• Apply an annual property tax increase (for example 2–4%).

Ignoring this means underestimating the true cost of ownership, especially when you compare it with renting, where you do not pay property tax directly.

Montant_travaux: the real hidden cost of old buildings (and sometimes new ones)

1. What montant_travaux covers in a buy or rent simulation

The montant_travaux parameter groups all the renovation and maintenance expenses over time:

• Immediate works at purchase (painting, updating kitchen/bathroom, wiring, energy upgrades).
• Planned medium-term works (roof, façade, boiler, windows, common areas).
• Mandatory works voted by the homeowners’ association.

For new builds, this amount is generally low at the beginning, but it should not be assumed to be zero over 20–25 years. For old properties, it can easily reach 5–30% of the purchase price, especially if a serious energy retrofit is needed after a poor energy rating.

2. Numerical example: new vs old at the same budget level

Assume you are comparing two options with a similar headline budget of €300,000:

• Option A – New build
  • Purchase price: €300,000.
  • Initial montant_travaux: €5,000 (kitchen fittings, minor adjustments).
  • Future works over 20 years: €15,000 (refresh, some co-ownership works).
  • Total works over 20 years: €20,000.
• Option B – Old building
  • Purchase price: €270,000 (discounted because it needs work).
  • Initial montant_travaux: €40,000 (insulation, windows, electrical upgrade, bathroom).
  • Future works over 20 years: €30,000 (roof share, façade, boiler replacement).
  • Total works over 20 years: €70,000.

On paper, the old flat looks €30,000 cheaper. But once you plug montant_travaux into the buy or rent simulator, the picture changes:

• Old: €270,000 + €70,000 = €340,000 total property cost over 20 years.
• New: €300,000 + €20,000 = €320,000.

And this is before you add notary fees (higher for old) and property tax. This is exactly the kind of gap a new vs old building simulation can reveal.

3. Energy rating and long-term renovation costs

The energy rating (DPE or equivalent) is a key driver for montant_travaux in older properties:

• A unit rated F or G can require €20,000–50,000 in energy retrofits to reach a decent level.
• These works can lower your heating bills, but they must be financed (loan, savings).

New builds that meet recent energy standards (e.g. RT 2012, RE 2020) typically offer:

• Much lower monthly energy bills.
• Limited need for heavy energy-related works in the first 15 years.

A serious buy or rent simulator should therefore let you set a higher montant_travaux for the old-building scenario and a lower one for the new-building scenario.

Comparing full scenarios: new vs old vs renting

1. Case study: household hesitating between three options

Consider a couple facing three realistic choices: stay tenants, buy a new flat, or buy an older one.

• Renting scenario
  • Current rent: €1,200/month.
  • Annual rent increase: 2.5%.
  • Available savings: €60,000 invested at 4% net (e.g. diversified ETFs).
• Buying new
  • Price: €320,000.
  • Notary fees: 3% = €9,600.
  • Agency fees: 4% = €12,800 (included in price or not, depending on the deal).
  • Initial montant_travaux: €10,000.
  • montant_fn: €0 for 2 years, then €1,000 per year, increased by 2.5% annually.
• Buying old
  • Price: €280,000.
  • Notary fees: 8% = €22,400.
  • Agency fees: 4% = €11,200.
  • Initial montant_travaux: €35,000 (including an energy upgrade).
  • montant_fn: €1,400 from year 1, increased by 3% annually.

Assuming a 3.6% mortgage rate over 25 years and 0.35% borrower insurance, the monthly payment for the new flat will be higher (because of the higher price). However, works and property tax will be lower initially. For the old flat, you face higher notary fees, larger works, and heavier property tax, but the purchase price is lower.

Alongside this, there is the opportunity cost: in the renting scenario, the down payment and any monthly surplus can stay invested at a 4% investment rate. A robust buy or rent simulation should therefore:

• Project your net wealth over time in all three scenarios.
• Include detailed montant_fn and montant_travaux for new and old buildings.

2. How montant_fn and montant_travaux change the winner

In theory, older properties often look more profitable because:

• The purchase price is lower.
• You might capture more upside after renovation.

But once you add:

• €10,000–20,000 in extra property tax over 20 years.
• €30,000–50,000 more in works compared to new builds.

The financial balance can flip in favour of new construction, or even in favour of staying a tenant while investing aggressively. This is why relying only on intuition for new vs old building choices is risky; a quantified buy or rent simulation is essential.

Investment perspective: real estate vs financial assets

Beyond the raw cost comparison, you should also consider how your capital works for you in each case:

• If you buy (new or old): your down payment is tied up in the property, you pay mortgage, property tax and works, but you gradually build housing equity.
• If you rent: you pay a rent that increases over time, but you can invest your savings at an assumed investment rate of, say, 3–6% per year depending on your risk appetite.

The outcome depends heavily on:

• The total montant_fn and montant_travaux in each ownership scenario.
• The actual performance of your investments vs inflation.
• Local property price trends for new and old buildings.

A good buy or rent tool shows your projected net wealth at the end of the period (for example 20 years) for each scenario: renting + investing, buying new, buying old.

Practical tips to set montant_fn and montant_travaux correctly

1. Estimating property tax (montant_fn)

• Ask for the latest property tax bill for the property (or a similar unit in the same building).
• Check the municipality’s track record on property tax hikes.
• In the simulator, apply a realistic annual increase (2–4% is a common range).
• For new builds, include any exemption period (set montant_fn to 0 or reduced for those years).

2. Estimating renovation and works (montant_travaux)

• Get several quotes for heavy works: insulation, windows, electrical, plumbing.
• Split your budget into immediate works and future works over 10–20 years.
• For old buildings with poor energy ratings, plan a substantial energy renovation budget.
• For new builds, don’t forget interior fittings (kitchen, storage, landscaping).

In a buy or rent simulator, you can either:

• Enter a single global montant_travaux as present value.
• Or, if the tool allows, schedule works at different years in the projection.

No universal answer: only numbers tailored to your case

There is no one-size-fits-all rule saying new builds are always better than old buildings or vice versa. The right choice for you, and the answer to the buy or rent question, depends entirely on your specific situation:

• City and local property tax level.
• Condition and energy rating of the old property.
• Price difference between new and old in your area.
• Mortgage terms, your savings, and how long you plan to stay.
• Your ability to invest spare cash if you keep renting.

This article is for information only and does not constitute personalised financial advice. To make a sound decision, the most reliable approach is a detailed simulation that includes realistic montant_fn and montant_travaux values in each scenario: new build, old building, and renting.

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