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Permits and budget · Plan the money · legal-7

Build a bottom-up cost plan

A bottom-up cost plan breaks the whole estate into parts and prices each part, so nothing is forgotten. Cost estimating guidance from the US Government Accountability Office (GAO) treats a detailed work breakdown structure as the basis of a reliable estimate.

How

  1. Create a work breakdown structure with one line per system: land and fees, site works and access, house shell, finishes, power, water, sanitation, heating and cooling, food and animals, waste, safety and communications, tools and equipment.
  2. Under each line list materials, labour, equipment hire, delivery, permits and professional fees.
  3. Price each item from real quotes, supplier price lists or recent local costs, not from national averages alone. Note the date and source of every price.
  4. Write down your assumptions (sizes, specification, who does the labour) next to each price.
  5. Flag the items you are least sure about; groundworks and ground conditions are commonly the hardest part to price in advance.
  6. Sanity-check the house against a benchmark. US example (NAHB 2024 survey): interior finishes 24.1%, rough-ins 19.2%, framing 16.6%, exterior finishes 13.4% of construction cost.
  7. The same survey gives foundations 10.5%, site work 7.6%, final steps 6.5% and other costs 2.1%. Treat these as shares, not prices.
  8. Off-grid systems (power, water, wastewater, storage) come on top of a conventional house split, and local quotes always rule.
  9. Safe room: US guidance (BASC, citing FEMA P-320) says a safe room may cost 5 to 10 percent of the home's cost. Price it as its own line (secure-4).
  10. Fix the design and specification before you start, because changes during the build tend to raise costs.
  11. Have the plan reviewed by someone independent, such as a quantity surveyor or estimator, if the budget is large.

Do these first

Sources

  1. Cost Estimating and Assessment Guide: cost estimating challenges. US Government Accountability Office, accessed 2026-11
  2. Cost Estimating and Assessment Guide: elements of cost estimate documentation. US Government Accountability Office, accessed 2026-11
  3. The costs of a self build project. National Self Build and Renovation Centre (NSBRC), accessed 2026-11
  4. Budgeting for your build project. Self Build Portal (NaCSBA), accessed 2026-11
  5. GAO-20-195G Cost Estimating and Assessment Guide (PDF). US Government Accountability Office (hosted by US Department of Energy), accessed 2026-11
  6. Top posts: cost of constructing a home in 2024 (NAHB Construction Cost Survey). National Association of Home Builders, Eye on Housing, accessed 2026-11
  7. Design for Windstorm and Tornado Protection (Building America Solution Center). Pacific Northwest National Laboratory for US DOE, accessed 2026-11

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