Ground-floor rear extension cost factors in Adelaide

On an established Adelaide block, site access to the rear of the home is often the single biggest cost driver. If the block has a wide side driveway that a bobcat, mini-excavator and concrete truck can reach the rear through, site works run cleanly and the trades can stage materials at the back of the home. If access is through a 900mm side passage between the home and the boundary fence, every cubic metre of concrete, every pallet of bricks and every stick of timber has to be carried by hand.

The impact of poor access is not linear. A hand-barrow site can add 15 to 25 percent to trade labour costs across the whole extension, because every subcontractor charges for the additional time. Some trades will decline the job entirely if access is too tight.

Get a signed site logistics plan from the builder before contract. It should specify the access route for each trade, the location of the site toilet and skip, the delivery windows for major materials, and any temporary fencing or protection of existing garden and paving.

Where the new extension roof meets the existing roof is a design and cost detail that plans almost never show properly. The roof tie-in typically requires removal of the existing roof cladding along the junction, structural bracing of the existing rafters or trusses to carry the new load, flashing to a high standard to prevent leaks, and a matching gutter and downpipe strategy.

On homes with a tile roof, the tie-in also involves sourcing matching tiles, which can be difficult with older Marseille or Cordoba profiles. On Colorbond homes the tie-in is cleaner but the colour match may fade over the next decade, so the builder should agree with you which strip of the original roof is replaced with the new material.

Ask the builder to itemise the roof tie-in as its own line in the quote. If it appears as a provisional sum, push back. A good specialist can price this to within a few percent, because they have done it many times.

The stormwater and sewer connection points on an Adelaide block are usually at the front boundary, in the street. If your rear extension adds a new bathroom, ensuite or kitchen, the plumbing must run from the rear of the extension all the way back to the front street connections, either along the side of the home or under the existing floor.

This trench work is disruptive, expensive, and rarely priced tightly in a first-pass quote. Expect 20 to 30 metres of new sewer trench and stormwater run for a modest rear extension in an inner suburb. If the existing home has combined sewer and stormwater, or if the property has an existing septic on some semi-rural blocks around Adelaide Hills and Onkaparinga, the plumbing scope changes further.

Insist that the plumbing scope shows the exact route, the exact depth, whether under-slab or above-slab, and whether any existing pipes need to be replaced en route because they are corroded or undersized.

The point at which the new extension meets the original home is where the most detailed craftsmanship is required, and where the largest hidden cost lives. Transition works include: bricking in old external openings, plaster patch and paint to affected walls, ceiling level matching if the extension has a higher ceiling than the existing home, floor level matching if the extension slab differs from the existing floor, electrical rewiring at the junction, and reworking of the existing HVAC to serve the new rooms.

On many quotes these items are lumped under a single line called "make good" or included as provisional sums. Do not accept that. Ask the builder to break each transition item into its own line with a fixed price. If the builder resists, ask a competing builder to quote the same job the same way. Any professional extension builder can and should itemise transition works.

The 15 to 25 percent range is a useful sanity check. If the transition items in a quote total less than 10 percent of the addition value, the builder is either underquoting or intending to charge back later as variations.

A provisional sum is an amount included in the contract as a placeholder because the exact scope is not yet known. In well-scoped extension contracts, provisional sums should account for less than 5 percent of the total contract value, and should apply only to items that genuinely cannot be pinned down before start on site, such as unknown asbestos removal in old wall linings or unknown foundation conditions revealed after opening up.

If your quote has provisional sums for kitchen benchtops, floor tiles, taps or paint colours, that is a red flag. Those items can and should be priced firmly at contract stage. If your quote has a large provisional sum labelled "structural allowance" or "electrical allowance", that is also a red flag, because both are quantifiable at design stage.

Before signing, sum every provisional item and ask the builder what happens if the actual cost exceeds the provisional. Under HIA and Master Builders SA standard contracts, cost overruns on provisional sums are payable by the homeowner. That is normal, but you need to know the exposure up front.