Across 80 sites I have supported in my practice since 2021, I have recorded an average final overrun of 14.3 % between the signed quote and the final invoice of a construction project. Not from commercial bad faith: because of items systematically forgotten or minimised in initial quotes. These 14 items represent on average between 25,000 and 60,000 € of extra cost on a 300,000 € house. Here is the exhaustive list of what most often returns in 2026, to be provisioned from the quote comparison phase.
The 5 items that sink the first months
Even before the site starts, several items escape the initial quote:
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Utility connections (water, gas, electricity, fibre): between 3,200 and 6,800 € in Wallonia, rarely included. See the article new build water electricity connection for details of steps and costs per operator.
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Town planning and notary fees post-permit. Mortgage release, cadastral registration fees, municipal taxes: 800 to 1,500 €.
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Soil studies mandatory since 2023 in Wallonia for any new construction. Count 850 to 1,400 € for a standard geotechnical analysis.
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Surveyor demarcation when the plot is not certified or the parcel boundary is disputed. 600 to 1,100 € depending on complexity.
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VAT at 21 % on undeveloped land, which many still confuse with the 6 % renovation rate. On land at 80,000 € exclusive of VAT, that represents 16,800 € of VAT — not a detail.
The 4 that emerge during the site
During execution, several extra costs emerge unpredictably:
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Polluted soil removal (analysis + accredited transport): easily 2,000 € if the land has an industrial or intensive agricultural history.
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Foundation rework if the soil study reveals reshaping requiring a raft or piles. From 4,000 to 18,000 € depending on the chosen solution.
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Material cost overrun 2026 index: the revision clause is almost always in the contract, and nobody reads it. In 2024-2025, the ABEX index rose by 3.2 %, i.e. about 9,000 € extra cost on a 280,000 € house.
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Plan modifications during execution: represent on average 8 % to 12 % of the initial budget on the sites I audit. Systematically underestimated by buyers.
The 5 that arrive after reception
The worst is yet to come after signing the minutes. Five items almost always emerge after moving in:
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Final EPC if the announced class is not met (insulation, airtightness, equipment corrections): 3,000 to 12,000 €.
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Rainwater tank under the new Breyne Law and RGBSR 2026 standards (5,000 L minimum) if not included in the contract: 2,800 to 4,500 €.
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Outdoor landscaping (paths, terrace, fence, gate) almost never priced in standard turnkey. See the article new house garden: 8,000 to 22,000 €.
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Fitted kitchen and custom storage: rarely included beyond a symbolic budget. 12,000 to 25,000 € for a standard fitted kitchen in 2026.
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Furniture and moving: systematically forgotten to provision. Count 8,000 to 15,000 € to minimally furnish a 4-bedroom house.
The total: an overrun of 25,000 to 60,000 €
Adding up these 14 items on an average project at 320,000 € excluding VAT (4-façade house, 180 m², Wallonia 2026), here is the estimate I calculate in practice:
- Pre-site phase: 9,000 to 17,000 €
- Site phase: 8,000 to 22,000 €
- Post-reception: 25,000 to 50,000 €
- Total probable overruns: 42,000 to 89,000 €
A realistic total budget between 362,000 and 409,000 € for a project announced at 320,000 €. This upfront honesty avoids site stoppages for lack of cash flow — which are the main cause of Breyne Law disputes I handle.
Pitfalls to avoid
- Don’t sign a contract without an exhaustive list of priced exclusions
- Check the indexation clause (index used, annual ceiling)
- Provision a minimum of 15 % beyond the quote
- Anticipate connection delays (12-25 days ORES, 6-12 weeks outside urban zones)
- Do not commit your personal contribution beyond 85 % of the loan: keep a margin
For official statistics on construction costs in Belgium, see statbel.fgov.be — construction and housing.
What’s next?
If you are in the quoting phase, have your offers reviewed by an expert before signature. My practice offers Breyne Law support that includes hunting missing items and pricing probable overruns, or request a free quote for a one-off budget audit.