How to build two-storey on a narrow infill lot in Adelaide

The Planning and Design Code overshadowing test in South Australia is typically applied to habitable-room windows and private open space of adjoining properties. A common misconception is that overshadowing is measured against the ground level of the neighbour block. In most Adelaide zones the test is stricter: it applies specifically to north-facing windows and to at least half of the neighbour private open space during specified hours.

For a two-storey narrow-block design, your designer must produce a shadow diagram at 9am, noon and 3pm on the September equinox (a standard test date). The diagram must show whether the shadow cast by the new home falls on any north-facing habitable window of the immediate neighbour.

If it does, the design either needs to be adjusted (lower ridge line, angled upper wall, set-back upper floor) or the application needs to be performance-assessed with a Statement of Effect explaining the impact. Public notification is likely in either case.

Overlooking is triggered where an upper-floor habitable room window sits within 3 metres of a neighbour habitable room window. The Code prescribes three acceptable responses: raise the sill height to at least 1500mm above the finished floor level (which prevents sitting-height overlooking); use permanently fixed obscure glazing below sill height 1500mm; or install external screening (fixed louvres or angled fins).

The high-sill approach is the cleanest architecturally but reduces natural light into the room. The obscure glazing approach is a functional compromise that most people accept for secondary rooms. The screening approach is often used for master bedrooms where a proper window is desired.

For narrow-block designs where every upper-floor room has at least one boundary within 3 metres, expect to deploy all three techniques across the plan. Discuss this with the designer at concept stage so it becomes a design feature rather than a retrofit.

Two-storey narrow-block designs work best with one of three stair strategies. First, a central switchback stair against a party wall. This uses the least floor area on both levels and delivers a compact plan. It works on blocks 8 metres wide or narrower.

Second, a rear straight-run stair adjacent to the garage. This preserves the front of the ground floor for the main living space, and works well on blocks 8 to 10 metres wide with generous depth.

Third, a feature stair as an internal focus point, typically a curved or open-tread stair visible from the entry. This eats more floor area but transforms the internal experience and is worth considering on blocks 9 metres and wider where floor area is not the binding constraint.

The stair type must be decided before the design goes past concept stage. Retrofitting a different stair type is a whole-plan redesign, not a small adjustment.

Roof form on a narrow-block two-storey has three drivers: block orientation for solar access, overshadowing to the neighbour, and material efficiency in construction.

The most common choice is a low-pitch gable running along the long axis of the block. This minimises overshadowing to the neighbour, allows generous room heights on the upper floor, and is efficient to frame with prefabricated roof trusses.

A skillion roof is a valid alternative and gives a strongly contemporary aesthetic. It works well where solar panels are a priority (a north-facing skillion of 15 degrees is close to optimal for solar generation in Adelaide) and where the block orientation allows.

A hipped roof (the classic suburban form) is usually a poor fit on narrow blocks. It reduces ceiling height at the eaves, complicates the framing, and eats floor area at the upper level. Avoid unless matching an existing character streetscape is a hard planning requirement.

Narrow-block sites often have restricted site access. Concrete pumps, cranes, and long-load deliveries may not be possible. This constrains the structural options.

For the upper floor, the two common approaches are traditional timber-frame construction (light, buildable by hand, no crane needed) and prefabricated modular panels (fast but requires crane access). On tight sites, timber-frame is usually the pragmatic choice even if slightly slower.

For the ground-floor slab, a waffle-pod slab is easier to pour on a tight site than a raft slab because it requires less concrete pump reach. Discuss the slab type with the structural engineer at design stage, not with the builder at pricing stage.

Steel members (posts, beams, upper-floor bearers) may be required in specific spans. Long steel members can be difficult to deliver to constrained sites and may need to be craned in through neighbour airspace. This is a real approvals conversation with council and adjoining owners, and it belongs on the design agenda from the start.