Site works are a most important part of the cost of development for any home building activity.
The activity includes earthworks, retaining walls, stormwater management, building pad shaping, fencing, access roads or paths, landscaping shapes. On large sites in difficult terrain site works can involve substantial costs and form a large percentage of the total cost of development. For this reason site works cost must be carefully considered in the overall cost of the project. For this reason the cost of any block being considered as a site for your new home must be assessed with the total site cost as part of the total price. So a standard small suburban block that is level and made entirely of sand is only going to involve a small earthworks cost before construction can start. But is you have a large steep block on clay soil, then the site works will be substantial and the total cost of the site works needs to be added to purchase price in order to compare it to other blocks. You will have noticed that most estate developments in Perth these days are designed to provide buyers with a flat, sandy building.
The Perth metropolitan area is well suited to building concrete and brick buildings as a large part of the plains on which most of Perth stands is sand. Sand is stable and easy to work and so site works costs in Perth are generally small compared with most other Australian cities. There are of course exceptions. The areas immediately adjacent to the Swan River are mostly clay, while in the hills we have rock and gravel. Site works costs in these areas will always be much higher than on a sand site. Additionally the engineering design for a house being constructed on a clay site is much more substantial leading to higher costs for both concrete and brickwork. Lastly a clay site also involves some difficulties in dealing with stormwater. Sand sites are managed by digging soak wells and connecting downpipes directly to the soak wells. Clay sites require surface water to be directed away from the house pad and may involve some subsoil drainage in order to deal with water moving through the subsoil.
Other factors that will impact site works costs are the presence of rock below the surface, the presence of peat or similar material below the surface and in older areas it is not uncommon to discover a long forgotten rubbish pit below the surface.
When you engage a builder to construct your home they will prepare a site works estimate. These estimates can vary widely depending on what of the items above have been included. It is most common for the estimate to be prepared based solely on a surface visual inspection. Now because the estimate is based on a surface inspection only it is presented a Provisional Sum in the quote and is subject to adjustment once the excavation has commenced. Because of the generally good building soils we have this is mostly adequate. However it is not uncommon for some rock to be discovered below the surface, or for some other unsuitable material to be found that in turn requires removal. Any unforeseen issues will usually involve additional cost.
If you have a site in a non-sand area then you need to ensure that a proper site investigation is carried out. In some cases a full geotechnical report will be necessary in order to properly understand your sub-soil. These investigations are expensive and take time. So builders do not normally carry these out prior to contract. If it is necessary on your site and you want to ensure the site works costs are known before you start then you will need to meet the cost of the investigation and report upfront. Some areas of Perth, for example Port Bouvard canals, the council requires a geotechnical report with every building license application. Once the geotechnical report has been received it will be used by the engineer on your project to design the structure of the building. In areas where a geotechnical report is required the resulting engineering design will usually involve some additional cost.
A geotechnical report is certainly a cost you should not under stake lightly and you need to discuss the site conditions with your builder to understand the risks involved in proceeding either way.
Builders operate in a competitive environment and will sometimes try to minimise the expected cost so as to make their price more attractive. They may simply leave necessary items out of the contract and place the responsibility for this work with you the client. The most common example of this is soak wells. They might assume a sandy site in an area known for having rock below the surface. They might put a standard cost allowance in upfront and note that an engineering design is yet to be completed. Most builders are professional in their approach to these matters and will give you good advice, but you need to make your own assessment of the possible problems your site might entail. If the builder you are talking too wants to skirt around the issues of your site costs be aware of the risk.
Now even with the best professional advice and even with a geotechnical report you will still sometime uncover something unexpected when construction begins. It is advisable to keep some budget amount available for unexpected items.
