How big should an AHU plant room be?

For a standard commercial office, allow 3 to 5 percent of net lettable area for mechanical plant rooms. For a single AHU room, the room area should be the AHU footprint multiplied by 2.0 to 2.5. A 7 sqm AHU footprint needs 14 to 18 sqm of room.

What are the AHU room dimensions for a 2,000 sqm office floor?

An AHU serving a 2,000 sqm office floor is typically 1.8 m wide by 4 to 5 m long, with a 7 to 9 sqm footprint. Plan an 18 to 25 sqm plant room with 3.6 m clear ceiling height, plus 1.5 to 2.0 sqm of adjacent riser shaft.

What clearances does an AHU need around it?

Allow 1000 to 1200 mm clear on the service side for filter, coil, and controls access; 600 to 900 mm on the non-service side; 1200 to 1500 mm at one end for coil and fan pull; and 600 to 900 mm above the highest connection for dampers and turning vanes.

What ceiling height does an AHU plant room need?

For mid-size AHUs, plan 3.2 to 4.0 m floor to underside of slab. Add 300 to 500 mm if the unit has an energy recovery wheel or humidifier. Headroom under any pipe, duct, or platform must be at least 2100 mm clear (AS 1657:2018, AS 1668.2).

What door size is needed for AHU replacement?

Personnel doors should be 900 mm wide by 2200 mm high. Coil and fan pass-through needs at least 1200 mm. For full unit replacement at the 15 to 20 year lifecycle, plan a dedicated equipment hatch, roll-up door, or removable wall panel sized 1500 to 2400 mm wide.

AHU Plant Room Size: 3-5% NLA Rule + Worked Example

Section 1

What You Need to Know

Lock in AHU plant room size at concept design. A standard commercial office needs 3% to 5% of net lettable area for mechanical plant. Get it wrong and you lose tenancy area later, or you cannot fit the unit at all.

The AHU plant room sets the structural grid, the riser layout, and the ceiling void depth. These are the hardest things to change after development application. If the room is too small, the contractor cannot change filters, pull coils, or replace the fan. The AHU has a 15 to 20 year life. The building has 50 plus. Plan for the second AHU swap, not just the first install.

Section 2

The Rules

Mechanical plant rooms occupy 3% to 5% of served net lettable area in standard commercial offices (industry rule of thumb)
Combined building services plant takes 5% to 8% of gross floor area in typical commercial buildings (industry rule of thumb)
Single AHU room area equals AHU footprint multiplied by 2.0 to 2.5, covering service access and duct transitions (AIRAH DA09 guidance)
Service-side clearance for filter, coil, and controls access: 1000 to 1200 mm clear (AIRAH DA09; AS 1668.2 access principles)
Non-service side clearance: 600 to 900 mm for inspection (industry standard)
End-of-unit pull space for coil or fan removal: 1200 to 1500 mm minimum (industry standard)
Overhead clearance above the highest connection: 600 to 900 mm for dampers, turning vanes, and BMS sensors (industry standard)
Mid-size AHU plant room ceiling height: 3.2 to 4.0 m clear floor to underside of slab (AIRAH DA09 guidance)
Add 300 to 500 mm ceiling height for energy recovery wheels, humidifiers, or stacked components (industry standard)
Headroom under any pipe, duct, or platform: 2100 mm clear minimum, 2400 mm preferred (AS 1657:2018, AS 1668.2)
Personnel door to plant room: 900 mm wide by 2200 mm high, opening outward (AS/NZS 3000 Cl 2.10.2.2 precedent)
Coil and fan pass-through door: 1200 mm minimum width to clear assemblies without disassembly (industry standard)
Full AHU replacement requires a dedicated equipment hatch, roll-up door, or removable wall panel sized 1500 to 2400 mm wide (industry standard)
Outdoor air intake must sit at least 6 m from any exhaust discharge of 1,000 L/s or more (AS 1668.2:2024)
Plant room ventilation: typical minimum 5 air changes per hour for general mechanical plant rooms (AS 1668.2)

Section 3

What This Means in Practice

Plant room size is set by two things: the AHU footprint and the access aisles around it. The AHU itself comes from the air volume calculation. A typical commercial office runs at 4 to 5 air changes per hour plus outdoor air. For a 2,000 m² floor that gives 5,500 to 7,000 L/s, which lands on a unit roughly 1.8 m wide by 4 to 5 m long.

Once you know the unit footprint (around 7 to 9 m²), apply the multiplier. Footprint x 2.0 to 2.5 gives the room area. That covers 1.0 m clear on the service side, 0.6 to 0.9 m on the back, end-pull space for coil and fan removal, and the duct and pipe transitions out of the room. So a 7 m² AHU needs 14 to 18 m² of room. Add ceiling height of 3.2 to 4.0 m and the room is set.

Co-locate the plant room with the supply, return, and outdoor air risers. Allow 1.5 to 2.0 m² of dedicated riser shaft per typical floor, immediately adjacent to the plant room. If the risers sit across the floor plate, ductwork eats ceiling void and tenancy net.

The single biggest mistake in early-stage design is treating plant rooms as leftover space. They are primary functional spaces. Lock the size, the ceiling height, and the equipment hatch location before the structural grid is signed off. Once the steel is up, the room cannot grow.

Section 4

Key Design Decisions

1

Floor Area Allocation

Allow 3% to 5% of NLA for mechanical plant in standard commercial offices. Healthcare and laboratory buildings need 8% to 15% of GFA. These are concept-stage targets, refined once the engineer runs the heat load.

Trade-off: shrink the room and you lose either cooling capacity or maintenance access. Both cost more across the building life than the lost lettable area.

2

Ceiling Height

Plant rooms are not normal rooms. Plan 3.2 to 4.0 m floor to soffit for mid-size AHUs. Add 300 to 500 mm if the unit has a heat recovery wheel or humidifier. Anything below 2400 mm only suits very small fan coil units in ceiling cupboards.

Trade-off: an extra 400 mm of slab-to-slab height per plant level adds facade and structural cost. Compress the plant room ceiling and you cannot lift the fan motor or service the upper coils.

3

Equipment Hatch for Replacement

The first AHU lasts 15 to 20 years. Replacing it through the lobby costs ten times more than a hatch. Put the equipment hatch in an external wall facing a vehicle bay, loading dock, or roof crane standing point. Size it for the largest single AHU component, typically 1500 to 2400 mm wide.

Trade-off: an equipment hatch costs $5,000 to $15,000 at build but saves $50,000 plus in crane hire and building damage when equipment is replaced.

4

Riser Co-location

Place the supply, return, and outdoor air risers immediately adjacent to the plant room, not across the floor plate. Allow 1.5 to 2.0 m² of riser shaft per typical office floor. Co-located risers keep ceiling void shallow and protect tenancy net floor area.

Trade-off: putting the plant room and risers on the perimeter loses external glazing along that wall. Internal locations protect the facade but require longer duct runs and deeper ceilings on adjacent floors.

Section 5

Who Needs to Know What

Worked Example

2,000 m² Office Floor

Cooling load: 110 to 125 kW (around 30 to 35 tons), based on 1 ton per 57 m² of typical office (industry rule of thumb)
Air volume: 5,500 to 7,000 L/s, covering 4 to 5 ACH plus outdoor air (AS 1668.2:2024)
AHU footprint: 7 to 9 m², around 1.8 m wide by 4 to 5 m long for a 6,000 L/s unit
Plant room area: 18 to 25 m², footprint multiplied by 2.0 to 2.5
Ceiling height: 3.6 m clear floor to soffit
Riser allowance: 1.5 to 2.0 m² immediately adjacent
Equipment hatch: 1500 mm wide by 2200 mm high in an external wall
Outdoor air intake: minimum 6 m horizontal separation from any exhaust discharge over 1,000 L/s (AS 1668.2:2024)

Total plant footprint at concept: allow 20 to 27 m² per floor, with riser space adjacent and a clear path from the hatch to a vehicle bay or roof crane location.

Section 6

References

National Construction Code 2022, Volume One — Class 5 to 9 building services provisions
AS 1668.2:2024, The use of ventilation and airconditioning in buildings — Part 2: Mechanical ventilation in buildings
AS 1657:2018, Fixed platforms, walkways, stairways and ladders — Design, construction and installation
AS/NZS 3000:2018 (Amd 2), Electrical installations (Wiring Rules), Clause 2.10.2.2
AS 4254 series, Ductwork for air-handling systems in buildings
AIRAH DA09, Air conditioning systems
AIRAH DA19, HVAC&R Maintenance