Two expressions for one stage
A frequent confusion is worth clearing at the outset. It concerns two expressions designating the same stage.
"Closed shell" and "weathertight" denote the same stage. They are not two successive steps but two ways of naming the same deliverable: the second describes the result, the first the scope of works.
Belgian commercial vocabulary uses both interchangeably, which sometimes suggests an additional intermediate stage that does not exist. There are nonetheless three stages and not four.
The four genuinely distinct stages
| Stage | What is delivered |
|---|---|
| Open shell | earthworks, foundations, slab, loadbearing masonry, roof structure |
| Closed shell | adds roof covering, external joinery and weather and air tightness |
| Part-finished shell | adds some services and finishes, scope varies |
| Turnkey | the whole, from earthworks to finishes, habitable on delivery |
The open shell does not put the building in the dry. Neither the complete roof covering nor the external joinery is installed, exposing the works and preventing any start on internal trades.
The closed shell is the stage most commonly priced in Belgian offers. It also gives meaning to the weathertight expression: the building is closed, covered and sealed, but entirely raw inside.
The part-finished shell has no stable definition. Its scope varies between contractors, and it is the stage producing most misunderstandings. It requires an explicit list of trades.
What the closed shell includes
The usual scope is: foundations, loadbearing masonry, ground slab, roof structure, roof covering, insulation and external joinery including windows and doors. That scope nonetheless varies between contractors and is checked in the quote.
External joinery weighs heavily in that total, in the order of ten to fifteen per cent, which explains part of the price dispersion: joinery quality and the performance level chosen move this item considerably. That item therefore deserves separate mention in any comparison.
At this stage no internal services are installed. No electrics, no plumbing, no heating, no plastering, no screeds, no finishes.
What share of the budget
Published sources diverge, and that should be stated rather than resolved. The gap between them lies in the scope each takes for complete finishing.
According to the references consulted, the closed shell represents between forty and sixty per cent of the full completion budget on some accounts, between fifty and seventy per cent on others. The wide band calls for checking the scope of each source.
That divergence is not a failure of rigour. It reflects three realities.
The content of the stage varies. Some contractors include insulation and thresholds, others do not.
The reference level of finish varies. A share expressed against a high-specification turnkey is mechanically lower than against a simple one.
The construction method varies. Timber frame and traditional brick do not distribute cost between structure and finishes in the same way.
Retaining a single figure would therefore mislead. Retain instead that roughly half the budget remains to be spent after the closed shell, and check the split on the actual project.
The scale effect
A regular phenomenon is worth knowing: the price per m² tends to fall slightly as area increases. The effect stays modest but it is constant.
The reason is mechanical. Foundations and roof do not double when habitable area doubles, whereas partitions and finishes track area far more closely.
Practical consequence: a benchmark observed on a small house does not transfer to a large one, and the error runs towards overstatement.
What choosing a stage transfers
Opting for a partial stage is not merely a budget decision. It is a transfer of burdens.
Coordination. Trades must follow one another in the right order. That coordination has a value, and it passes from contractor to client.
Interface liability. A defect at the junction between two trades carried out by two separate firms raises a question of attribution that does not arise under a single contract.
Warranties. Works carried out outside the main contract do not fall under the same cover. This point deserves checking before the decision.
Carrying cost. A project spread over several phases lengthens the period during which the client bears both rent and finance.
The apparent saving of a partial stage must therefore be set against those four burdens, not read as a net gain. The client in fact takes on whatever the contractor does not execute.
What this means for an estimate
Three rules.
Ask for the list of trades rather than the name of the stage. The name is not enough, particularly for the part-finished shell.
Check the treatment of insulation and joinery. These are the two items whose inclusion or exclusion moves the benchmark most.
Never compare a closed-shell benchmark with a turnkey one. The gap between them describes no difference in quality.
This article reflects professional practice at the date of checking and serves as orientation. It does not constitute legal advice and does not replace assessment of the individual case.