Ventilation cannot be added later
A boiler room has two distinct air requirements: combustion air and ventilation air. When the two are conflated the outcome is the same — the appliance runs inefficiently and, in the worse case, safety devices trip and the building goes cold.
Grille areas are sized from appliance capacity and provided at two levels, low and high. If these openings are not in the architectural drawings, correcting them later means cutting through finished concrete — the most expensive fix on the list.
Service clearance takes as much room as the appliance
When laying out a boiler room the appliance footprint is often taken as sufficient. In practice you need clearance to withdraw the burner, swing the boiler door, change the pump and reach the expansion vessel. Without it every service visit takes longer, and some jobs require dismantling other equipment first.
There is also the door: the boiler that went in will one day need to come out. If the wall was built after the boiler was placed, replacing the unit means taking the wall down.
Gas detectors go at the right height
Natural gas is lighter than air and rises when it leaks, so its detector sits near the ceiling. LPG is heavier and settles at floor level, so its detector goes low. Where both fuels are present in one room, you need both.
A detector on its own is not enough: unless it is wired to the solenoid valve and the extract fan it only sounds an alarm without cutting the gas. Once wired, the system must be tested and the test recorded.
Floor and drainage get forgotten
There is water in a boiler room: discharge from the safety valve, condensate from a condensing appliance, filling and draining operations. Without a floor gully that water ends up under the electrical panel or the pump.
Condensate is acidic and needs neutralising before it reaches the drain. Floor falls and gully positions are decided before the boiler room slab is poured — they are not something you can add afterwards.