Granny flat build timeline: what happens week by week

Construction of a granny flat typically runs 12 to 16 weeks from site set-up to handover. The stages happen in a fixed order because each one depends on the last, and knowing that order tells you what you are looking at when you glance out the back window and what you need to have decided by then.

This is a typical sequence. Weather, inspections and site conditions move it around.

Site fencing goes up, access is established, and the ground is prepared. Excavation levels the building area, and service trenches are dug for water, sewer, power and data.

This is the stage where surprises appear. Rock, unexpected soil conditions, or a service in an unexpected position all show up now rather than later.

What you need decided: everything about siting and levels. Changing the position of the building after excavation is expensive.

Formwork is set out, reinforcing steel is placed, and the under-slab plumbing is installed. An inspection happens before the pour, then the concrete goes down and cures.

The slab is a hard commitment point. Plumbing positions are set in concrete, literally.

What you need decided: bathroom, kitchen and laundry layouts, because the drainage positions are fixed here.

The frame goes up and suddenly there is a building. Roof trusses follow, then roofing.

This is the stage where people finally get a feel for the size of the rooms, and where the occasional change of heart happens. Changes here are still possible but they are no longer cheap.

What you need decided: window and door positions, which are framed now.

Windows and external doors are installed and the external cladding goes on. Once the building can be locked, materials can be stored inside and weather stops holding things up.

Lock-up is a common progress payment stage.

Electrical cabling and plumbing pipework are run through the wall and ceiling cavities before they are closed up. Inspections happen at this point.

What you need decided: every power point, light switch, light fitting and tap position. This is the last easy moment to change them. After the walls are lined, moving a power point means cutting into plasterboard.

Insulation goes in, then wall and ceiling linings, then cornices and skirtings. Kitchen and bathroom cabinetry is installed and wet area tiling is done.

The building starts to look finished from the inside.

What you need decided: all finishes, because they are being installed. Tiles, cabinetry, benchtops and tapware should have been selected weeks earlier.

Painting, floor coverings, and the installation of fixtures and fittings. Appliances go in. The building becomes a home rather than a shell.

Driveway and paths, any fencing, landscaping and turf. Final plumbing and electrical connections are completed and tested.

Then final inspections, the occupation certificate, and handover.

What happens at handover: you walk through the building with your builder, note any defects, and receive the keys along with your certification and warranty documentation.

Looking at the sequence, the same pattern shows up repeatedly. The stages that stall are almost always waiting on a decision rather than on work.

Making all of these before construction starts is the single most effective thing you can do to keep a build on schedule.

• Weather. Mostly affects the early stages. Excavation and the slab pour need reasonably dry conditions. Once the roof is on, it matters much less.
• Inspections. Several stages require an inspection before the next can start.
• Rock or unexpected site conditions. Discovered during excavation.
• Variations. Every change you request has a time cost as well as a price.
• Material or trade availability. Outside anyone’s control, though a builder with steady trades manages it better.