Extension Costs & Timelines: A Practical Guide

A home extension is one of the most effective ways to add space and value without moving house — but it's also a project where cost and timeline estimates can vary wildly depending on a handful of key factors. Here's what actually drives them.

What drives the cost of an extension

• Footprint and storey count. A single-storey extension is generally more cost-effective per square metre than a second-storey addition, which needs additional structural support for the existing home.
• Site access and slope. A tight block, steep slope, or limited truck/crane access adds cost before a single wall goes up.
• Services and connections. Extending plumbing, electrical and (if applicable) gas lines into the new space, and tying the extension's roof and drainage into the existing house, is often more complex than it looks from a floor plan.
• Matching the existing home. Matching brick, cladding, roofline and window styles so the extension looks like it was always part of the house can cost more than a visually "different" addition — but it's almost always worth it for resale value and street appeal.
• Finish level. Kitchens, bathrooms and living areas at the higher end of the finish spectrum move the budget more than any other single factor.

What drives the schedule

• Council approval time. This is the step most homeowners underestimate. Depending on the council and whether the extension is a straightforward "as of right" build or needs a planning permit (for example, due to overlays, heritage controls, or boundary setbacks), approval can take anywhere from a few weeks to several months.
• Weather and site conditions. Groundwork and slab pours are weather-dependent; a wet Melbourne winter can push a schedule out.
• Long-lead items. Custom windows, specific tiles, or imported fixtures can have lead times of many weeks — ordering early, once designs are locked in, keeps the build on schedule.
• Living arrangements. Extensions built while a family remains in the house typically take longer than a vacant-site build, since work has to be sequenced around daily life.

A realistic planning window

For a typical single-storey extension in Melbourne's eastern suburbs, homeowners should plan for roughly 2–4 months from initial design to a lodged permit application, then a further 3–5 months of construction once approved — though every site is different. The best way to get numbers specific to your block is a site visit and a straightforward conversation, not a rough online calculator.

Register your extension project with EBC for a free consultation, and we'll walk you through a realistic cost and timeline for your specific site.