Construction in occupied premises: transforming without interrupting your business
Four critical interfaces we arbitrate simultaneously
Works on an occupied site cost 8 to 12% more than a vacant site, but temporarily vacating a floor plate costs 3 to 6 months of avoided rent: the economic trade-off almost always favours maintained occupancy, except below 300 m2. The vast majority of our projects today take place on occupied sites, a share that has been rising markedly for several years. The trade-off logic differs radically from a vacant site: every technical decision, from the layout of pathways to the scheduling of utility shutdowns, factors in the human constraint. Our practice since 2006 confirms an operational finding: the quality of project management on occupied sites is determined before the start, in mapping the incompatibilities between core business activity and works sequences.
Working on an active floor plate requires resolving four structuring tensions, each measurable and manageable.
- Temporal interface: calibrating phases according to core business activity cycles (accounting close, sales peaks, sensitive HR periods), using staggered hours from 6 pm to 11 pm or extended weekends.
- Spatial interface: delimiting zones through airtight airlocks and mobile acoustic partitions, in reference to the requirements of the NF S 31-080 standard on acoustic levels in offices.
- Logistical interface: material flows separated from occupant circulation (deliveries before 8 am, dedicated goods lifts, buffer storage in the basement).
- Safety interface: permanent maintenance of exits in accordance with Labour Code R4227, access to critical equipment (main LV switchboard, network racks, smoke extraction).
Poor anticipation of these interfaces generates 15 to 25% cost overruns and 3 to 6 weeks of slippage. The fundamental trade-off consists of balancing execution speed and minimising disruption, according to the criticality of the hosted activity and the hourly value of the employees concerned.
Contrary to industry doxa that overvalues execution speed, our reading differs: on an occupied site, accelerating the works by 2 weeks costs more than extending the schedule by 4 weeks with reduced shutdowns. The extra cost of staggered hours, additional labour and permanent monitoring exceeds the avoided-rent gain as long as operations continue. The right metric is not the raw timeline, but the total cost including disruption.
When this four-interface framework does not apply. Below 300 m2 renovated on a floor plate with fewer than 30 occupants, the four-interface formalism becomes oversized: mapping the incompatibilities takes more time than the works themselves. In that case, simply vacating the floor plate temporarily for 3 to 5 working days costs less than the phasing engineering. The method becomes fully relevant from 500 m2 or whenever the hosted activity precludes any closure window (trading floor, continuous-flow call centre, laboratory).
