Factors That Affect Site Costs

Sloping Sites and Block Layout:

Having a levelled block for your new custom home can significantly reduce site costs since there is less excavation and spoil removal and often no need for retaining walls. However, many property owners neglect to factor in stormwater hydraulic works required for their site. A level block can be advantageous for stormwater lines running directly from the rainwater tank to the street. However, if there isn’t sufficient fall (i.e., min 1% grade), it can create challenges. This is known as a “charged line”, and most councils do not accept charged systems. This may not impact single dwellings, but most councils require duplexes to have on-site detention such as underground tanks, above ground OSD, and offset retention/detention volume within rainwater tanks.
Additional consideration is required for blocks that slope to the rear—they often have increased costs associated with them compared to blocks that slope frontwards. Typically, rear-sloping blocks require an easement (sometimes through neighbouring properties), which increases site costs.

Wind and Soil

The condition of the soil and soil type on your block can also increase site costs. Consider the following factors before proceeding with your custom-built home:
• Sandy soil: A site with sandy soil will require “screw piles”, an anchoring system used to secure deep or unstable foundations. Due to the loose soil profile, screw piles will generally need to be installed much deeper than denser clay-based soil. This ensures your building has the necessary bearing capacity.
• Rocky soil: Encountering rocks on your site can potentially increase site costs due to the high cost of machinery hire and the length of time required to break and remove rocks from your property.
• Soil classification: Soil is generally categorised in classes such as A, S, M, H1, H2 and E. These classifications are based on site reactivity to moisture and ground movement. M-Class slabs require smaller foundation pods, which use less concrete overall. On the other hand, H-Class slabs require larger pods and strip beams—foundational requirements that will impact site costs. A structural engineer will determine the suitable slab type based on your Wind and Soil Report.
See the Abstract below from the Building Code of Australia (BCA).
• Marine, salinity, and acid sulphate: Saline/acid sulphate-affected sites or blocks located within a marine environment require various upgrades to enable construction. Your block will require upgrades depending on your site’s proximity to breaking surf, sheltered bays, and other marine environments. Site enhancements may include galvanised steel, epoxy zinc treatment of structural steel, marine grade bricks, upgrades to mortar mixes, stainless steel brick-ties, and potential upgrades to colour bond fascia and gutters. In addition, saline/acid sulphate-affected sites will require upgrades to the concrete strength (e.g., 32MPa in-lieu of 20MPa), higher-grade dampproof membranes, and a 50mm sand bed.
When you purchase a land block, it is highly recommended that you obtain a Wind and Soil Report (Zee Builders will include this as part of your custom home building contract).

Bushfire

Before purchasing a block, you need to review the site’s 88B or conduct research to determine if your site is bushfire affected. In Sydney, several local councils are well-known for being located within bushfire affected zones. If your site is situated in a bushfire zone, you need to obtain a bushfire report to determine the severity and proximity to bushfire. The report will categorise your property into one of the following five zones:
• BAL–LOW – No special Construction requirements
• BAL–12.5
• BAL–19
• BAL–40
• BAL–FZ
See Table below for upgrade requirements.
Sites located within BAL–12.5 zones will require minor upgrades such as doors, window seals, amber protection, and metal mesh screens which can add $7-10K (based on a standard single storey home).
Sites located within BAL–40 or BAL–FZ zones are the most problematic. They face stringent and extensive requirements, some of which can cost upwards of $40K to satisfy. If you wish to avoid these additional costs, Zee Builders advises that you steer clear of land that is prone to bushfires.

Flood

As the recent floods in Sydney have demonstrated, it’s essential to consider the extent of your site being flood affected. If your block of land is within a flood-affected area, your local council may request flood reports that can cost up to $3500. Where necessary, the council will also impose special conditions, such as suspended floors, finished floor level restrictions, clear side access, rainwater tanks on elevated pads, services and electrical installed at heights above flood levels, etc. Budgeting for a flood-affected site can be very challenging since it is dependent on the extent of risk. Some sites may only require minor adjustments to plans and floor levels to meet “1 in 100-year” flood levels—other sites may need a major redesign and changes to construction methods and materials.

S73 and Statutory Requirements

When purchasing a block of land, it’s highly recommended to review the sewer diagram and 88B instrument. Some sites may have sewer mains running on the site or directly through the block. This can be problematic since it would require modification to building methods. Building over a sewer, for example, requires concrete encasement, bridging beams, and other structural changes. Building adjacent to sewers requires the building to be clear of the “no piering zone” (900mm from sewer centre line) and any piers within the sewer main “zone of influence” to be piered a minimum of 300mm below the invert of the sewer. Other statutory requirements that can affect the site costs include (but are not limited to): gas, water, electricity, NBN, and power poles.

Whether you’re budgeting for a new custom home or a knockdown rebuild project, it’s essential to consider all possible site conditions since it tends to be a major culprit in cost overruns. To help guide your budget allocations and give you a more accurate estimate of project costs, Zee Builders strongly recommends that you obtain as much information as possible about your site.