What a rear extension is
A rear extension adds floor area to the back of the house, usually into the garden. It might be a single room, or it might replace a small kitchen and dining area with one larger space opening to the yard. A builder's guide lists rear extensions as one of the most common extension types in Adelaide, along with side extensions, second storeys and wraparound additions.
Soil and footings
A builder's guide says much of Adelaide sits on reactive clay that expands when wet and shrinks when dry, and that footing design has to allow for it to avoid cracking. It says this can mean deeper footings, engineered slabs or pier-and-beam foundations, depending on the soil classification. Ask each builder whether a soil test is included, who designs the footings and what happens to the price if the test shows a problem.
Joining new to old
The same guide says one of the biggest challenges in any extension is the junction between old and new construction, and that Adelaide homes span sandstone and bluestone in the inner suburbs, double brick from the 1950s to 1970s and lighter framing in newer areas. Ask how the builder would match your walls and roofline, how the new roof will be flashed to the old one, and how the floor levels will line up. Ask to see photographs of the join, not just the finished room.
Light, heat and the garden
Adelaide summers can exceed 40 degrees, according to the guide, which suggests deep eaves on the north and west, windows that open for cross-ventilation and suitable glazing. Think about where the sun falls on the new room and how much of the garden you are willing to give up. A rear extension also changes how you use the yard, so check where the clothesline, tank, trees and drains sit before the design is fixed.
What it costs
Published figures for a single-storey rear extension are $2,800 to $4,500 per square metre including design, engineering, approvals and standard finishes in one builder's guide, and $2,500 to $3,700 for construction only in another. A single-room addition of about 40 square metres is given as $100,000 to $150,000 by the second guide. Treat them as a way to frame a conversation and not as a price.
Approvals
A builder's guide says extensions need approval under the Planning and Design Code, and that those meeting standard setback, height and site-coverage rules are assessed more quickly than those that vary from them. PlanSA points owners to code.plan.sa.gov.au. Ask your council what applies and ask each builder who lodges the application.
Licence and insurance
Ask for the building work contractor licence details and check it on the SA licence register, and ask to see the building indemnity insurance certificate if the work needs council approval and costs $20,000 or more.
What to put in your brief
A rear extension is easiest to price when the brief says what the room is for. A kitchen needs services and ventilation, a bedroom needs a wardrobe and a window, and a living room needs good light. Mark where you want doors to the garden, how you want to get from the old house into the new space and what you are willing to remove, such as a bathroom, laundry or a window that would be built over.
Existing services and the yard
Before the design is fixed, ask each builder to look for sewer lines, stormwater pipes, hot water units, air conditioners and meters near the area you want to build over. Moving a pipe or a meter can add cost and delay. Ask what the builder would do about trees, fences and retaining walls, and whether the quote includes making good the garden afterwards.
Staging and living there
Many owners stay in the house during a rear extension, at least until the new work is joined on. Ask how the builder will keep dust out of the rest of the house, where they will store materials, how the power and water will be turned off and for how long, and how they will protect floors and furniture. If the join to the old house is going to disturb the kitchen, ask how long you will be without it.
How much does a rear extension cost in Adelaide?
Published 2026 guides give $2,800 to $4,500 per square metre including design and approvals, and $2,500 to $3,700 for construction only. They are not quotes. Ask each builder what is included.
Does Adelaide clay affect an extension?
A builder's guide says reactive clay can move with moisture, so footings need to allow for it. Ask who designs the footings and whether a soil test is included.
How do I keep the new room cool?
A builder's guide suggests deep eaves, cross-ventilation and suitable glazing. Ask each builder or designer how the room will face the sun.
Who applies for approval?
Usually the builder or designer prepares and lodges it, but check. Ask who lodges it and whether the fees are in the quote.