Design Considerations / Cost & Budget / Setting the budget

Site costs

The same house can cost very different amounts on two different blocks. What's in the ground, how the land falls, what has to come down first, the trees you have to keep and the services running through the site all show up in the price. Most of it can be found out before design starts, and it should be.

About 5 minutes · 7 sections

At a glance

Start with what the free online maps already show: contours, boundaries, trees, easements and services. Allow for the ground, the slope, the trees and the services in the budget from the start, and assume an older house has asbestos.

  • Canberra's clay is reactive, and the hill suburbs can hold rock. Both affect footings and excavation.
  • Demolishing a house disturbs the ground. The site often gets a worse soil classification, and the new house may need piers.
  • Waffle slabs suit flat and gently sloping blocks. Real crossfall means stepping the slab or a suspended floor, which then needs extra insulation and sealing.
  • Fixed asbestos in the eaves and cladding of older homes is a routine cost of demolition, not a surprise.
  • Designing around regulated trees can make the house more complex, and removing them can be hard.
  • Touch the services and the authorities will charge: new power goes underground, and easements and access for water and sewer have to be kept clear.

The groundWhat's underneath

Before a house can be engineered, the ground has to be classified. A soil test sorts the site by how much the soil moves as it wets and dries, and whether it's been disturbed. That classification decides the footings, and the footings are one of the biggest site costs.

Clay. Much of Canberra sits on reactive clay that swells when wet and shrinks when dry. It's manageable, and most houses are designed for it, but the more reactive the clay, the stiffer and deeper the footings need to be.

Rock. In the hill suburbs, excavation can hit rock. Breaking it out takes time and heavy equipment, and its extent is hard to know until digging starts.

Disturbed ground. Knocking down a house changes the site. Pulling out the old footings, filling the holes and churning the ground often gives the site a worse classification than an untouched block. The new house may then need piers down to undisturbed ground, and how deep they go depends on how deep the old footings were.

The slopeWhen the block falls away

Most new houses in the region sit on a waffle slab, a concrete slab cast over a grid of polystyrene pods. It's quick and economical on flat or gently sloping ground.

Once there's real crossfall across the site, the slab has to be stepped, with changes of level built into the floor, or the house moves to a suspended timber floor on piers or stumps. Either adds cost. Cutting the block flat brings its own costs: retaining walls, drainage behind them and carting the spoil away.

A suspended floor also changes the energy rating. Air can move under it, so it loses more heat than a slab on the ground. It needs good insulation and careful sealing to make up the difference, and that belongs in the budget too.

DemolitionClearing the old house

A knockdown starts before the demolition crew arrives.

  • Asbestos. Houses built before about 1990 often contain fixed asbestos in the eave linings, wall cladding and wet-area linings. It has to be identified by a survey and removed by a licensed contractor before demolition. In an older house, this is a routine part of the cost, not an unexpected extra.
  • Disconnecting the services. Power, water, sewer and any gas supply have to be properly disconnected or made safe, and each authority has its own process and fees.
  • The ground afterwards. As above, demolition often worsens the soil classification and can add piers to the new house.

TreesDesigning around what has to stay

Larger trees may be regulated, and can't be removed or damaged without approval. Approval to remove one isn't guaranteed and can take time.

Keeping a regulated tree means keeping clear of its protection zone. Footings, excavation, driveways and even site access have to stay out of the area around its trunk and roots. Designing around that zone can push the house into a more complex form, with an L-shape, a narrower footprint, a step back from the tree or special footings near it, and complexity costs money. Find out early which trees are regulated, because they can shape the whole plan.

Touch it and someone will charge for it

Servicing a single house is usually straightforward. But once a project touches any of the services, each authority involved will have requirements, and usually a fee. New power to the house will generally need to come in underground, even where the old supply was overhead. Water, sewer and stormwater mains running through the block sit in easements, which have to be kept clear, with access maintained for the authority that owns them. Building near a sewer main can mean deeper footings or piers, so the main isn't loaded. For dual occupancies and small multi-unit sites, servicing becomes a much bigger subject in its own right.

Water, access and set-upThe rest of the site

  • Stormwater. If the block falls away from the street, roof water may not drain to the street by gravity. That can mean a pump, a tank or an easement through a neighbour's land.
  • Access. Narrow side access, a steep driveway or a tight street can mean smaller machines, more labour, a crane or traffic management.
  • Neighbours. Excavating close to a boundary may need the neighbour's footings protected, and a record of the condition of their house before work starts.
  • Site set-up. Fencing, temporary power and toilet, erosion and sediment controls, and protection of the verge and footpath are part of every job.

Finding outBefore design, and as it firms up

A surprising amount is known before the first sketch, at no cost. The ACT and NSW governments' free online map viewers, along with satellite imagery, show the contours, boundaries, trees, easements and where the services run. That's enough to start designing with the main site constraints in view, and usually enough to see which trees are likely to be a problem.

The detailed investigations follow as the design firms up:

  1. A detail survey, once the design is under way, records the levels, boundaries, neighbouring buildings, and the size and position of the trees precisely.
  2. A soil report, which the engineer needs to design the footings. On a knockdown, it's best done after demolition.
  3. An asbestos survey for any older house to be demolished or renovated. Assume there's asbestos until it shows otherwise. The survey will be needed anyway, and the builder normally arranges it closer to demolition.
  4. An arborist's assessment, where a tree is likely to be regulated or sits close to the works.

Bringing a builder in early helps here too. A builder who has walked the site can price access, demolition and excavation far more reliably than an allowance in a spreadsheet.

Applying it locallyCanberra and New South Wales

Canberra. Reactive clay is common, and rock is more likely in the hill suburbs. Many knockdown sites are older houses with fixed asbestos, and new electricity supply generally has to go underground. Water, sewer and stormwater mains often cross older blocks, with easements to keep clear.

New South Wales. Councils set their own tree rules, and water authorities differ from area to area. Sloping coastal and escarpment sites can need extensive retaining and engineered footings, and some carry landslip or flood requirements.

Rural blocks. Long power runs, water storage, on-site wastewater treatment, access roads and bushfire requirements can each be major costs, and none of them is in a builder's base price.