Most budget blowouts on new homes aren't caused by one disaster. They're baked in before the slab is poured: a design that was never affordable, a quote with gaps in it, and a string of "while we're at it" upgrades. Here are the three things that make the biggest difference.

Tip 1: Set your budget before you design

The most common mistake I see is people falling in love with a design before they know what they can afford. Work out your total spend first, with your lender, and treat it as a cap.

Then make sure the budget covers the costs people forget:

  • Site costs: soil test, slope, rock, retaining walls and fill. A sloping or reactive site can add tens of thousands.
  • Provisional sums and prime cost (PC) items: allowances in the contract for work or fittings not yet priced. If the allowance is too low, you pay the difference.
  • Council, certification and service connection fees: water, sewer, power and NBN.
  • The outside: driveway, fencing, landscaping, letterbox, clothesline. Often excluded from the contract, always needed before you can live there.
  • Window coverings, appliances and anything else marked "by owner".
  • A contingency of 10 to 15% that you don't touch unless you have to.

Design to the budget, not the other way around. Things that keep costs down without making the house worse:

  • A simple, rectangular footprint. Every extra corner adds framing, roofing and labour.
  • Standard ceiling heights and standard window sizes.
  • Fewer wet areas. Bathrooms and laundries are the most expensive rooms per square metre.
  • A clear list of must-haves and nice-to-haves, so you know what to drop if the quotes come in high.

Tip 2: Get builder quotes you can actually compare

Most homeowners sign one fixed-price contract with a builder, so the quotes that matter are builder quotes. Get at least three on the same plans and the same specification, then compare them line by line:

  • What's included and what's excluded.
  • How many provisional sums there are, and whether the allowances are realistic.
  • The PC item allowances for things like tapware, tiles and appliances.
  • Site costs: fixed price or provisional?

If one quote is much lower, find out why. Usually it's because something has been left out or under-allowed, and you'll pay for it later as a variation.

Pick the builder whose quote is complete, whose QBCC licence and financial category check out on the QBCC licence search, and who answers your questions in writing. A builder you get along with is nice; a builder whose quote holds up is better.

Before you sign, ask for any changes you already know you want to be priced and written into the contract. Variations after signing are almost always dearer. My Asking the Right Questions checklist (C4) at constructor.net.au/checklists covers what to ask before you commit.

Tip 3: Control variations and payments during the build

Once construction starts, the budget is mostly lost through variations and careless payments.

Variations

  • Get every change priced and agreed in writing before the work is done. Under Queensland law a variation to a domestic building contract must be in writing.
  • Ask what else a change affects. Moving a window can mean new lintels, new bracing and a revised energy rating.
  • Keep a running total of approved variations against your contingency.

Payments

  • In Queensland the deposit is capped by law: 5% for contracts of $20,000 or more, 10% under that.
  • Progress claims are paid by stage under your contract (base, frame, enclosed, fixing, practical completion). Never pay a claim for a stage that isn't actually finished.
  • Get an independent inspection at key stages so you know the work matches the claim.
  • Check provisional sum and PC adjustments on each claim against the allowances in your contract.

Stay involved

Visit the site regularly, raise questions early and keep everything in writing. It's much easier to sort out a problem, or a cost, before the work is covered up.

The short version

  • Budget for the whole job, including site costs, outside works and a 10 to 15% contingency.
  • Compare at least three builder quotes on the same plans, line by line.
  • Get every variation priced in writing and only pay for stages that are complete.