Published 20 October 2026 · 6 minute read
On most apartment sites the basement car park is the single biggest variable in construction cost. Every level means excavation, shoring, a structural slab, ventilation and fire services, and the number of levels is decided by how efficiently you can fit bays onto the site. Here are the basics every early layout should respect.
Bay and aisle sizes
In Australia, off-street car parks are designed to AS/NZS 2890.1. For residential and long-term parking (user class 1A) the familiar figures for 90-degree parking are:
- Bay: 2.4 m wide by 5.4 m long.
- Aisle: 5.8 m wide for two-way traffic.
- Headroom: at least 2.2 m to any obstruction (more over accessible bays).
Put two rows of bays either side of one aisle and you get the basic module: 5.4 + 5.8 + 5.4 = 16.6 m. A site whose basement width is a multiple of that module (plus a little for walls and a cross aisle) parks efficiently; a site 12 m wide wastes a lot of area. Always check the current edition of the standard and your council's requirements, which can be stricter.
The rule of thumb: 25 to 35 m² per space
Divide the basement area by the number of bays and well-planned residential basements usually come out somewhere between 25 and 35 m² per space once aisles, ramps, cores, walls and storage are included. Below that, check the aisles really connect; above it, the shape of the site or the position of the cores is costing you bays.
Ramps
The ramp has to start at the street frontage, which is why the designer asks you to mark one edge as the frontage. AS/NZS 2890.1 limits ramp gradients (steeper grades are allowed for short private ramps than for public car parks) and requires transitions, typically around 2 m long, where the grade changes sharply so cars do not scrape. With a level-to-level height of about 3 m, a straight ramp plus its transitions needs roughly 18 to 22 m of length, and it takes that length away from parking on every level it serves.
Accessible bays and storage
Accessible parking (AS/NZS 2890.6) uses a 2.4 m bay with a 2.4 m shared zone beside it and needs extra headroom, so place these bays close to the lift cores. Apartment storage cages are often placed in leftover space at the ends of bay rows, where a full bay does not fit.
What decides the number of levels
- Demand: the number of apartments by type times your council's rates, plus visitor and retail spaces.
- Bays per level: the basement area less the ramp, cores and walls, divided by the area each bay needs.
- Levels = demand divided by bays per level, rounded up. Going from 2 to 3 levels adds roughly 50 percent to the excavation, shoring and basement structure.
How AI Will Do That CAD lays out a basement
The basement boundary is the site inset by the basement setback you set. The planner places the ramp at the frontage (trying several positions and orientations), runs a spine aisle from it, then tries every position of the 16.6 m module on two grid alignments and keeps the one with the most bays. Bays may not overlap the lift cores, the bays nearest the cores become accessible bays, and storage cages fill leftover space along the aisles. It then adds levels until your parking demand is met, up to the maximum you allow, and prices excavation against the real terrain.
Five ways to get more bays per level
- Align the parking grid with the longest site edge instead of the buildings (Criteria, Basement parking, Grid alignment).
- Move the ramp along the frontage so the spine aisle serves both halves of the site.
- Reduce the basement setback where deep soil and neighbours allow.
- Use buildings whose cores land on the line of the aisles.
- Check whether lower parking rates apply near public transport.
A generated basement layout is a concept for estimating bays and cost. A traffic engineer must design and certify the car park.
Next: a 10-minute feasibility workflow, or open the sample site and look at its basement in the Floors view.