Linear renovation projection

How we calculate — a guide for property owners

A year-by-year forecast of what your building is worth technically, how far it
has worn, and what it would cost to bring it back to good condition. It is
called linear because the wear it applies is a straight line — the same number
of percentage points every single year — and that one decision shapes everything
else in this document.

The short version

• Wear is a straight line. One fixed rate, applied every year, from the year

the building went up. No curve, no acceleration.

• A renovation changes the slope, not the shape. The line steps up, then

carries on straight — at a slightly steeper constant rate.

• One row per year, eleven years. This year plus the next ten, always.
• Renovation debt is measured against 75%. What it costs to get back to good

condition, not to new.

Contents

1. A straight line, by design
2. The four numbers in every row
3. What you get back
4. Plan a renovation
5. A renovation changes the slope, not the shape
6. Renovation debt and the 75% mark
7. Where the line stops
8. Every figure is in that year's money
9. Questions we get asked
10. In summary

1. A straight line, by design

One wear rate. The same every year, at every age, for the whole building.

This calculation treats your building as a single object with a single lifespan,
and wears it down at a constant rate. Two numbers set that rate:

• Building lifecycle — how many years the building is reckoned to last, say 50.
• Unwearable portion — the share that never wears away at all, typically 30%.

Foundations, site work and basic structure remain whatever happens.

Annual wear = ( 100% − unwearable portion ) ÷ building lifecycle

For a 50-year building with a 30% unwearable portion:
    ( 100% − 30% ) ÷ 50 = 1.4 percentage points per year

That rate never changes. A five-year-old building and a forty-five-year-old
building both lose exactly 1.4 points in the coming year. Written out, the
projection is as plain as it sounds:

Every step is identical. That is the whole of the linear model — **everything
else in this document is a consequence of it.**

What the straight line gives up, and what it buys

Real building parts do not wear like this. A new roof barely changes in its first
years and deteriorates quickly in its last; the honest shape is a curve that
starts flat and steepens. Our part-by-part calculation uses exactly that curve,
because for a single roof we know its age and its lifespan precisely.

The linear model deliberately gives that up. Here is the same building, aged both
ways — same starting point, same lifespan, same floor:

       condition
100% |***......
     |  ****   .......
     |     ****      .....
 85% |        ****       .....
     |           *****       ....
     |               ****       ....
 70% |                  ****       ...
     |                     *****     ...
     |                         ****     ...
 55% |                            ****    ...
     |                               ****   ...
     |                                   **** ..
 40% |                                      ****..
     |                                         ****.
     |                                            ************  ← 30% floor
 25% |                                            ^
     +--------------------------------------------|-----------
      0        10       20        30       40      50       60   years since built
                                                  |
                                       lifecycle ends (50 yrs)

     ****  linear model (straight)      ....  part-level model (curve)

Same start, same end, different middle. Both reach the 30% floor at 50 years.
Between those points the straight line sits below the curve — at 25 years it
reads 65% where the curve reads 82.5%. The linear model is the more cautious of
the two through most of a building's life.

What it buys in exchange is comparability. Because the slope is a single
constant, you can move a renovation five years earlier and read the consequence
immediately, with no part-level data and nothing hidden in the shape of a curve.
Every euro and every year enters the answer in a way you can check by hand.

Which one should I trust?

Both, for different questions. Use the part-by-part condition figures to
judge where the building stands today — they know each component's real age and
real renewal cost. Use this linear projection to compare plans against each
other over the next decade.

Expect the two to be close but not identical. If the linear figure looks
pessimistic against the part-level one, that is the straight line doing what
the chart above shows.

2. The four numbers in every row

Two in euros, one as a percentage, one the cost of catching up.

Condition and technical value are the same fact twice. **One as a percentage,
one in euros.**

3. What you get back

One row per year, running from today to ten years out.

This projection answers a planning question rather than a survey question: *if
nothing changes, where is this building in ten years — and what does it look like
if we renovate instead?*

Every row covers one year and carries the four figures above. Renovations you
have entered appear in the year you planned them.

Where the line starts

The forecast always ends ten years from now. Where it starts depends on what
you have given it:

So if you record a condition assessment from 2019, you get the full history from
2019 forward as well as the forecast. Give it nothing and you get today onwards.

Either way the straight line itself runs from the year the building was built —
you are choosing how much of it to be shown, not changing its slope. The one
thing that does change the line is supplying a measured condition: from that
year on, the line continues from your figure instead of from the calculation's
own estimate.

4. Plan a renovation

Move the money and the year, and all four numbers respond.

Below is a worked building: completed in 2005, a 50-year lifecycle, a 30%
unwearable portion, and a replacement cost of €4.2 million today. Its condition
this year is 72%.

The building here is a worked example chosen to be easy to follow — it is not
taken from any real property.

Do nothing

€700k in 2030

€1.4M in 2029

€1.4M in 2035

Compare the last two: the same €1.4 million, spent six years apart. Section 5
works through what that difference costs you.

5. A renovation changes the slope, not the shape

The line steps up, then carries on straight — a little steeper than before.

In the year you spend the money, it is added to the building's technical value.
Spend €1.4 million and the technical value rises by €1.4 million; the condition
rises by whatever share of that year's replacement cost the sum represents. One
step up, in one year.

After that the line resumes — and it is still a straight line. This is worth
stating plainly, because it is what keeps the model predictable:

A renovation never bends the line. **It raises it, then sets a new constant
rate of decline.**

Why the new slope is steeper

The renovation is now part of the building, and it wears at the same annual rate
as everything else. So the yearly decline is no longer just the building's basic
wear — it is that, plus the wear on what you added. The two add together into a
single new constant.

Here is the worked example, before and after a €1.4 million renovation in 2029:

Exactly 1.40 points a year before, exactly 1.84 after. Not drifting, not
curving — one constant replaced by another. Drawn out, the condition line looks
like this: straight, one vertical step, then a slightly steeper straight run.

100% |              ●───────
     |               ╲──────────
     |                      ╲────── new rate, −1.84/yr
 85% |
     |
 70% |●────
     |     ╲──── base rate, −1.40/yr
     |          ╲
 55% |               (do nothing)
     +-------------------------------
      2026    2029      2032    2036
              ↑ €1.4M renovation

This is not a penalty, and it does not undo the renovation — the building is far
better off than it would have been. It simply means **a renovation buys a step
up, not a permanently flatter slope.**

Sooner or later?

Compare the last two tables in section 4. The same €1.4 million, spent in 2029 or
in 2035, lands the building in almost the same place by 2036 — 86.3% against
85.5%. What differs is everything in between. Spend in 2029 and the building
passes six years in good condition with no renovation debt at all. Spend in 2035
and you carry a debt that grows past €780,000 before it clears.

The endpoints barely differ; the journey differs enormously. That is exactly the
comparison this projection exists to let you make, and it is not one you can make
from a single year's figures.

6. Renovation debt and the 75% mark

The cost of catching up — measured against good condition, not against new.

Renovation debt is the money it would take to bring the building back to
75% condition:

Renovation debt = replacement cost × ( 75% − condition )

…and never less than zero.

75% rather than 100% is a deliberate choice. A building kept permanently at 100%
would mean replacing things the moment they showed any wear, which nobody does
and nobody should. 75% is the level at which a building is in good,
well-maintained shape with no meaningful backlog.

Two consequences follow. A building above 75% has zero renovation debt — not
a negative one; the figure simply stops at zero. And because the debt is
calculated against the replacement cost in that year, it grows over time even
when the condition is unchanged, because construction costs rise.

Worth knowing when you budget

A renovation debt of €900,000 in 2036 is stated in 2036 money. If you compare
it against a budget written in today's euros, you are comparing two different
things — and the gap is bigger than it looks.

7. Where the line stops

*The straight line runs down to 30% condition and then goes flat, however long
you leave it.*

No matter how many years pass without maintenance, this projection will not
report the building below 30% condition. When the straight line would take it
lower, the condition is held at 30% and the technical value is set to the
unwearable portion of the replacement cost.

The reasoning is the same as at part level: a neglected building is not worth
nothing. The structure, the foundations and the site are still there, and they
still have value. Below a certain point the calculation stops trying to draw a
distinction it cannot really make.

In practice this matters most for older buildings that have had little work done.
If you see the line sitting flat at exactly 30% year after year, that is the
floor, not a plateau — the real position may be somewhat worse, and a part-level
assessment will tell you more than this projection can.

8. Every figure is in that year's money

Euros are restated year by year with a construction-cost index.

Construction costs rise. A projection that quoted every year in today's euros
would quietly understate what a renovation in 2034 actually costs. So every euro
figure in every row is expressed in the prices of its own year, carried forward
with a construction-cost index.

This produces one result that surprises people, and it is worth being ready for
it:

The technical value in euros can rise while the condition falls.

Both are true at once. The building is wearing out — condition drops every year
without fail. But construction costs may be rising faster than the building
wears, so the euro figure attached to it climbs anyway. Nothing has gone wrong,
and the building has not improved.

The practical rule: **read condition to judge the building, and euros to size a
budget.** Comparing this year's euros to a figure from five years ago will
mislead you; comparing this year's condition to five years ago will not.

9. Questions we get asked

Why does the projection always stop ten years out?

Ten years is the horizon over which a maintenance plan is worth making. Beyond
that, the wear assumption and the cost index are both doing more work than the
underlying data can support, and the numbers would look more precise than they
really are.

I entered a renovation but the condition barely moved. Why?

The condition gain is the renovation divided by that year's replacement cost. On
a €4.2 million building, a €200,000 renovation is under 5 percentage points. The
debt figure is usually the more useful readout for smaller jobs — it moves euro
for euro.

Why is my renovation debt zero when the building clearly needs work?

The building is above 75% condition, so by this measure there is no backlog. It
does not mean nothing needs doing — a single failing component can sit inside a
building that is comfortably above 75% overall. Renovation debt is a
whole-building catch-up figure, not a to-do list; the part-level figures are
where individual problems show up.

Why does condition fall faster after a renovation?

Because the new work is now part of the building and wears at the same rate as
the rest of it. From the renovation year onward the annual decline includes wear
on what you added. Section 5 works through this.

Can a renovation push the condition above 100%?

Arithmetically yes, if you enter more than it would cost to renew the worn part
of the building. The projection does not cap it, so a figure above 100% is a
signal that the renovation you entered is larger than the building has room for —
worth re-checking the amount rather than reading it as a real result.

Can I enter several renovations?

Yes — renovations are recorded per year, so you can plan a programme across
several years and each one is added in its own year. The examples in section 4
take one at a time to keep the effect legible, but the calculation itself has no
such limit.

Why doesn't this match the part-by-part condition figures?

They are two different models answering two different questions. This one is
linear — a single building lifecycle and one constant rate — so you can compare
plans quickly. The part-level calculation gives every component its own
accelerating curve and its own renewal cost, so it is more accurate about where
you stand today.

The chart in section 1 shows exactly how far apart they can be in the middle
years. Expect them to be close but not identical, and prefer the part-level
figures whenever the question is about the present.

What if I have a proper condition assessment?

Enter it with the year it applies to, and the projection will start there and run
forward from your figure rather than from its own estimate. A recorded technical
value in euros works the same way. This is the single most effective thing you
can do to make the forecast worth trusting.

In summary

1. The model is linear: one constant wear rate, applied to the whole building, at

every age.

2. You get one row per year: this year through ten years out, or further back if

you supplied a dated condition or technical value.

3. Each row carries replacement cost, technical value, condition and renovation

debt.

4. Wear is a straight line — one fixed number of percentage points a year, set by

the building's lifecycle and its unwearable portion. It never accelerates.

5. A renovation adds its full cost to technical value in the year you spend it,

in one step.

6. After that the line is straight again, at a new and slightly steeper constant

rate, because the new work wears too.

7. Renovation debt is the cost of getting back to 75% condition, and is zero

above it.

8. Condition never falls below 30%, however long the building is left.
9. Every euro figure is stated in its own year's prices, so euros can rise while

condition falls.

The building in the worked example was chosen to be easy to follow — it is not
taken from any real property. Your own lifecycle, unwearable portion, replacement
cost and starting condition come from your building's own data.

Companion document: Building Condition Explained,
which covers how the part, group and whole-building condition percentages are
calculated.

Updated on: 18/09/2026