The surprises on a new build are rarely cheap. Rock under the slab, old fill nobody mentioned, a sewer line in the wrong spot: each one can mean a variation, a delay and a hard conversation with the bank. You can't eliminate every surprise, but you can shrink the list a lot before you sign a contract.

A handy way to think about it is the old idea of knowns and unknowns, made famous by US Defence Secretary Donald Rumsfeld in 2002. It sorts what you know about your build into four boxes.

The four boxes

  • Known knowns: things you know and have allowed for. The house design, the contract price, the block size.
  • Known unknowns: things you know you don't know yet. You know there might be rock; you just don't know how much.
  • Unknown knowns: things you know but haven't applied. You read that the block was once a dam site, then forgot about it when you approved the slab design.
  • Unknown unknowns: the things nobody saw coming. An old septic tank under the driveway, or a buried stormwater pipe that isn't on any plan.

The aim is simple: move as much as you can into the first two boxes before construction starts, and keep money aside for what's left.

Common unknowns on a house block

  • Rock. Shows up during excavation and footings. Breaking it out takes time and bigger machines.
  • Old or uncontrolled fill. Common on infill blocks and some estates. It can need deeper footings, piers or extra engineering.
  • Reactive clay. Affects the slab design and site classification.
  • Groundwater and poor drainage. Wet sites can need extra drainage, a higher slab or more careful footings.
  • Buried services. Sewer, stormwater, water, power, gas and telecommunications, some of them old and badly recorded.
  • Trees and roots. Large trees nearby, or ones recently removed, can affect footing design.
  • Easements and overlays. Flood, bushfire, vegetation or heritage overlays can change what you can build and where.

Turning unknowns into knowns

Most of these can be found, or at least narrowed down, with a few inexpensive steps before you sign:

  1. Get a soil test (geotechnical report). It tells the engineer how reactive the soil is and whether there's fill, rock or water near the surface. Ask where the bore holes were taken. If they're in one corner and the house sits somewhere else, that's a gap.
  2. Get a contour and detail survey. It shows the slope, levels, existing services, trees and boundaries, which drive cut and fill and retaining costs.
  3. Check the property search and plans. Easements, sewer locations and council overlays are on record. A Before You Dig Australia enquiry shows what's underground nearby.
  4. Walk the block. Look for signs of old fill, wet patches, rock outcrops, old pits or concrete, and ask the neighbours what they found when they built.
  5. Read the contract's site cost clauses. Look for rock clauses, provisional sums and exclusions. Ask the builder what happens if they hit rock, and how it will be measured and priced.

Our C1 Before Buying Land checklist covers these checks, and it's free.

The unknown knowns

This box is the sneaky one. It's the information you already had but didn't use: the soil report you never read, the sales brochure that mentioned "engineered fill", the neighbour who told you about the flooding. Before you sign, sit down with every report and document you have and ask, "Does my contract and design actually allow for this?"

Experience is often the same. A good site supervisor can tell by the feel of the excavator bucket that the ground isn't right, even if they couldn't explain exactly why. Listen when your builder or engineer raises a concern.

Planning for what's left

No amount of testing finds everything. That's why you need a contingency: money set aside in your budget, not already committed to upgrades. I'd suggest something in the range of 10 to 15% of the build cost for a sloping or older infill block, and at least 5 to 10% on a flat, well-documented estate block. Tell your lender about it early so it's part of your approval, not an afterthought.

If you don't need it, you'll have money for landscaping. If you do, you'll be glad it was there.

Further reading

Understanding Surveys & Contour Plans: A guide 4 homeownersA contour survey maps land elevation,important for new homeowners in planning and construction. Understand RP and SP roles, contour lines, and reduced levels. This guide helps interpret survey plans for accurate design, drainage, and compliance.man and woman walking on green grass field during daytimeBuilder or Land First? Key Tips for Starting Your Dream HomeEmbark on building your dream home with a strategic approach. This guide helps you decide whether to choose a builder or land first, weighing the pros and cons of each option. It covers cost considerations, availability, timelines, and expert advice for a smooth home-building process.