How to estimate renovation costs: the interior designer's method (+ 2026 prices)

Why cost estimation is where projects are won or lost

In our conversations with interior designers, the quote comes up again and again as the profession's number-one pain point, even before project tracking. The reason is simple: a bad estimate costs you twice. Too high, and you lose the project. Too low, and the end of the job turns into a running negotiation over change orders, at the expense of the client relationship and your margin.

The good news: estimating is a skill of method more than intuition. It can be structured.

The 6-step method

1. Thorough site survey and assessment. Actual surfaces (not the ones in the listing), the condition of what's already there (electrical, plumbing, substrates), access constraints (floor level, elevator, parking). Anything you don't survey, you'll discover on site, the hard way.
2. Breakdown into trade packages. Demolition, structural work, plumbing, electrical, insulation, partitions/plasterwork, joinery, floor and wall coverings, painting, finishes. This granularity is what lets you compare contractors' bids and manage options. It is also what turns your cost plan into a tendering template that contractors can price line by line.
3. Quantities for each trade. The take-offs: square meters to paint (walls + ceilings, not floor area), linear meters of baseboards, number of electrical points… A wrong quantity makes even the best unit price useless.
4. Applying reference prices. This is where the freshness of your data makes the difference, see the ranges below. Adjust each price to the specifics of the job rather than applying a national average.
5. Explicit margins and contingencies. Build 5–10% technical contingency into the estimate, and recommend in writing that your client set aside an overall reserve of 15–20%. Making uncertainty visible protects the relationship, hiding it destroys it.
6. Formatting the quote. Numbered versions, the right VAT rate on each line (10% for improvement work, 20% in certain cases, 5.5% for energy-efficiency work), options kept separate from the base scope. Great substance never rescues an unreadable format, we covered this point in our guide to quoting software for interior designers.

2026 reference prices (Paris region)

Excerpts from the Nodal pricing library, 2026-08 vintage, 403 work items across 14 trades, prices in € excl. VAT supplied-and-installed, cross-checked against public price guides and updated every month. Every job is specific, these figures are a starting point, not a selling price.

To give your clients a rough overall figure before getting into the detail of each trade, our guide to renovating an apartment in Paris lays out the per-square-meter ranges by type of renovation.

Tools & templates: what do you actually need to estimate?

Three families of tools cover most of the need: a well-built spreadsheet to get started, reliable price references to feed it, and dedicated software once project volume makes the spreadsheet unmanageable. Let's look at all three honestly, limits included.

The spreadsheet template that works

A spreadsheet estimate (Excel or Google Sheets) is still where most studios start, and there's no shame in that: well structured, it produces perfectly serious estimates. The template that has proven itself holds in three tabs:

• "Survey" tab: one row per room, with wall / floor / ceiling areas, heights, singular points (ducts, sill walls, existing networks) and referenced photos. This is the raw material of your quantities — everything starts here.
• "Estimate" tab: one row per work item, grouped by trade package, with seven fixed columns: trade, item description, unit (m², linear meter, unit, lump sum), quantity, unit price excl. VAT, VAT rate, total. One row = one work item; never a "miscellaneous" or "package" row — those are what make quotes impossible to compare.
• "Summary" tab: totals per trade, subtotal excl. VAT, a technical contingency line (5–10%), VAT broken out by rate, total incl. VAT. This is the only tab the client sees.

Three usage rules prevent the classic accidents. Freeze every version you send (a dated PDF, or a named copy of the tab) and never touch a version once sent. Lock the formula cells, so only quantities and prices get typed. Finally, date your unit prices: an undated price is a dead price — you'll never know whether it came from a 2024 job or last month's catalog.

Example: the template filled in for an 8 m² bathroom

Here's what the estimate tab looks like on a simplified real case — a complete 8 m² bathroom in an older Parisian building (Nodal database prices 2026-08, € excl. VAT, supplied and installed):

The point of the example is not the total, it's the structure. Every line can be checked, discussed and compared with a contractor's offer. A client who finds the budget high then challenges one specific line, not the whole thing: the conversation stays technical, and it stays healthy.

This template is also the one we recommend to educators: trade-by-trade estimating is precisely one of the professional acts that curricula rarely cover, as we detail in our guide to building a "professional tools" module in interior design schools.

What the spreadsheet can't do

The spreadsheet hits its limits exactly where the job gets complicated:

• Prices age in silence. Nobody revisits 400 unit prices every quarter; the reference base drifts, and every estimate drifts with it.
• Formulas break without warning. A row inserted in the wrong place, a sum range that shifted: the error stays invisible until the invoice.
• No real version history. "estimate-v3-final-2.xlsx" is not versioning. In a dispute, proving what the client approved, and when, becomes acrobatic.
• No client approval. A spreadsheet produces an amount, not a time-stamped agreement.
• No bridge to the tender. Everything has to be re-typed to build the bill of quantities you'll use to consult contractors.

Where to find reference prices when you're starting out

The template is empty without prices to fill it, and this is where young studios lose the most time. Four sources, always cross-checked:

• Your own site feedback. The best source, and the only truly local one: every contractor quote you receive, every final invoice is a real, dated, contextualized price. Archive them by trade from your very first project.
• Public price guides (annual or online). Useful to frame an order of magnitude, but their national averages need adjusting: allow +15 to +30% in the Paris region on most trades.
• Quotes requested directly. On a work item you don't know (a custom glass partition, a specific screed), nothing replaces two calls to contractors: fifteen minutes that prevent a 40% error.
• A maintained pricing library. The option that removes the monitoring work altogether: a database of construction work items updated continuously, whose values you adjust to your context instead of starting from zero.

Whatever the source, the rule is the same: a price is always recorded with its date and its context. Three dated, localized prices are worth more than thirty anonymous averages.

From spreadsheet to Nodal: the tooled-up estimate

The method in this article has a hidden cost: keeping prices current. Combing through supplier sites and cross-checking feedback from the field, project after project, takes hours every month, and it's the first thing interior designers drop when work picks up. The result: price lists that age, and estimates that drift.

That's exactly the problem a connected pricing library solves. In Nodal, every quote line draws on a real construction pricing database, updated every month, and every line stays editable, because your knowledge of the job has the final say. Concretely, the workflow follows the method described above: the survey feeds the quantities, every line is pulled from the library (403 work items across 14 trades) then adjusted to context, VAT is broken out automatically at the right rate, and the quote goes out for online client approval, time-stamped. The same trade-by-trade breakdown then comes back out as a tender template, with no re-typing. We described what that changes day to day in this article on coordination.

The 6 estimating mistakes that cost the most

After reviewing dozens of estimates, the same mistakes keep coming back, and almost all of them are made before the first unit price:

1. Estimating from the listing's floor area. The advertised area is neither the true floor area nor, above all, the developed wall area to be painted. Pricing paint on floor area — the most common mistake — understates that trade by 2.5 to 3 times.
2. Forgetting site setup and rubble removal. Protecting common areas, hoisting, skips or rubble bags, end-of-job cleaning: 3 to 8% of the budget which, if not estimated, gets paid through change orders or out of your margin.
3. Applying a national average to a Parisian job. A France-wide average price on a job with no elevator, an occupied unit, and 1890s walls: that's a 15–30% structural gap, in the wrong direction.
4. Burying contingency in the unit prices. Inflating every line "to be safe" makes the estimate impossible to compare or understand. Contingency is its own line, explained to the client — it's what protects the relationship when the surprise shows up.
5. Leaving all VAT at 20%. On a home more than two years old, the 10% / 20% split (and 5.5% for energy work) changes the total by thousands of euros. Detailing it line by line also signals competence — we devoted a full guide to it.
6. Sending an amount without a scope. A total without a list of what's excluded (furniture, appliances, engineering studies, fees) will always be read as "all-inclusive". Exclusions go in writing, on the estimate itself.

Frequently asked questions