The 7 mistakes at a glance

  1. Underestimating the real budget (with no contingency reserve)
  2. Choosing the cheapest tradesperson without any checks
  3. Doing the trades in the wrong order
  4. Starting work with no plans or specifications
  5. Changing your mind mid-project
  6. Neglecting the administrative side (co-ownership, city hall)
  7. Coordinating the tradespeople on your own

Underestimating the budget — with no reserve set aside

This is the most common and most expensive mistake. Most owners base their budget on estimates found online or on prices given by a friend "who knows about this stuff." The result: an initial budget that is too low, discoveries during the work (asbestos, worn-out plumbing, out-of-square walls), and financial stress that sabotages the whole project. For realistic ranges, see our guide to renovation prices in Paris.

Tip: Always add a reserve of 15 to 20% to your estimated budget. This is not pessimism — it is rigor. On an €80,000 project, this €12,000 to €16,000 reserve keeps you from having to halt the job halfway through.

Choosing the cheapest tradesperson without any checks

The temptation is strong. A quote 30% below the others looks like a bargain. It almost always hides a reality: lower-quality materials, undeclared subcontracting, no ten-year warranty insurance, or a tradesperson who abandons the job before it is finished. The cost of redoing a botched job is far higher than what you "saved."

Tip: Always verify the ten-year warranty insurance (Kbis company registration + insurance certificate), ask for 2 to 3 verifiable references, and be wary of any tradesperson who refuses to sign a detailed quote.

Doing the trades in the wrong order

Laying the wood flooring before the plumbing work. Painting the walls before finishing the electrical. Tiling before the wall chases are done. These sequencing mistakes force you to demolish and redo — at your own expense. The correct order of the trades is not intuitive and varies with the layout of the job.

Tip: The standard order is: demolition → structural work → rough plumbing → rough electrical → insulation → partition walls → plastering → joinery → tiling → wood flooring → paint → plumbing finishing → electrical finishing. Any deviation should be justified by a professional.

Starting work with no plans or specifications

A quote with no plan is a blank check. "We'll figure it out on site" is the phrase that precedes most job-site disputes. Without precise specifications, each tradesperson interprets the instructions their own way. And when the results do not match your expectations, recourse is extremely difficult without contractual documentation.

Tip: A dimensioned 1:50 plan, an exhaustive list of materials with references, and a provisional schedule should be attached to every quote before signing. Producing these documents is precisely the role of an interior designer or a project manager.

Changing your mind mid-project

You had planned a 6 m² bathroom and you ask the worker to expand it to 8 m² "while we're at it." Every change during the work generates extra jobs, wasted materials, longer timelines, and often tension with the tradespeople. Change orders are billed at rush rates — count on 30 to 50% more than if they had been planned.

Tip: Take the time you need before signing. Visit showrooms, build a precise moodboard, and only start the work once you are 95% sure of your choices. The remaining 5% is for the unforeseeable — not for changes of heart.

Neglecting the administrative side

Work started without co-ownership approval, a façade change with no prior declaration, creating living space with no building permit… These oversights can lead to work stoppages, fines, or even an obligation to restore the premises at your own expense.

Tip: Check your co-ownership rules and your building manager before any work. For significant work, an architect or a project manager systematically verifies administrative compliance. Two weeks of paperwork beats six months of proceedings.

Coordinating the tradespeople on your own

This is probably the decision that generates the most stress and overruns. Coordinating 8 different trades, each with its own constraints and its own schedule, requires an availability and an expertise that most owners do not have. A plumber who is not there on the agreed day blocks the electrician, who blocks the tiler, who blocks the painter…

A poorly coordinated job can fall several weeks behind on a three-month project — with all the costs that implies: rent on a temporary place, furniture storage, interest on the loan…
Tip: Delegate the coordination to a professional — an interior designer or a project manager. The cost of this delegation (generally 8 to 15% of the cost of the work) is most often recouped through the savings generated on timelines and the mistakes avoided.

In summary: what separates successful projects from disastrous ones

Projects that go well are not the ones whose owners "got lucky." They are the ones where the owner invested time upfront — in defining the project precisely, choosing tradespeople rigorously, and organizing the coordination. Or, better still, the ones where they delegated that organization to a qualified professional.

And the level of tooling of that professional matters: an interior designer equipped with management software like Nodal gives you a real-time tracking space — job-site milestones, quotes to approve online, centralized messaging — instead of a project followed through scattered emails and calls. Transparency is no longer an effort; it becomes a default setting.

Frequently asked questions

What budget reserve should you plan for a renovation?

Always plan a reserve of 15 to 20% on top of your estimated budget. On an €80,000 project, this €12,000 to €16,000 reserve absorbs on-site surprises (asbestos, worn-out plumbing, out-of-square walls) without stopping the work.

In what order should you carry out renovation work?

The standard order is: demolition, structural work, rough plumbing, rough electrical, insulation, partition walls, plastering, joinery, tiling, wood flooring, paint, then plumbing and electrical finishing. Any deviation should be justified by a professional.

Who can coordinate the job site for me?

An interior designer or a project manager (maître d'œuvre). The cost of this delegation (generally 8 to 15% of the cost of the work) is most often recouped through the savings made on timelines and the mistakes avoided.

Are you an interior designer?

You already know these mistakes — your clients don't. To equip your practice: our method for costing renovation work and our guide to quoting software for interior designers.