Engineering Due Diligence Before You Buy A Development Site

Before you buy a development site, engineering due diligence should tell you what the ground, the water and the infrastructure under and around it will cost to build over. A feasibility that assumes a standard basement on an unknown site is a guess with a spreadsheet attached.

The expensive surprises are rarely well hidden. A sewer main through the middle of the lot or a flood affectation usually sits on a public record that nobody read before the deposit was paid.

Why it belongs before the contract

Once the contract is signed, every engineering finding becomes your cost. Before that, the same finding is a reason to negotiate on price, or to walk away.

The ground

The first question is what the basement will be dug into. A desktop review of the geological map, nearby investigations and the site's history gives a first answer, whether that is sandstone, shale, clay, sand or old fill.

If the vendor allows access, a few boreholes before exchange are worth more than any desktop study. They show the depth to rock and the soil profile, and a standpipe left in one of them starts telling you about groundwater. Numbers and placement are covered in [how many boreholes a development site needs](/blog/how-many-boreholes-does-a-development-site-need).

Groundwater

Groundwater changes a basement more than almost any other finding. A basement below the water table brings shoring that holds back water as well as soil, a structure designed for uplift, and the approvals that come with taking groundwater, and in sandy ground like the Botany Sands all of it arrives at once.

The council maps

The maps attached to the local environmental plan tell you a great deal at no cost. Check the acid sulfate soils class, flood planning, heritage items and conservation areas, and any landslip or geotechnical hazard mapping, then read the planning certificate for the lot.

An [acid sulfate soils](/blog/acid-sulfate-soils-on-sydney-development-sites) class, a flood affectation or a heritage neighbour rarely stops a project on its own. Each one adds reports, design work and time, and they belong in the numbers before you bid.

Sydney Water assets

Sewer and water mains often run through the back of residential lots, and they are easy to miss on a site inspection. A main under the planned basement means relocating it, building over it with approval, or designing around it.

Before You Dig Australia and Sydney Water's own records show where the assets are. A building close to one, or over it, brings a Specialist Engineering Assessment, as set out in [building over sewer mains in Sydney](/blog/building-over-sewer-mains-sydney-engineering-pathways).

Stormwater and the fall of the land

A site that falls to the street drains easily. A site that falls to the rear needs somewhere for its stormwater to go, usually an easement through a neighbour, and that answer isn't in your control.

Overland flow is the other check. A lot sitting on an [overland flow path](/blog/overland-flow-paths-through-a-development-site) carries raised floors, a high driveway crest and rules about fencing, and all of them land on the architecture.

Transport corridors

A site beside a railway, a Metro line or a state road brings the transport authority into the excavation and the piling. That means an engineering assessment report, movement limits on the shoring and monitoring through construction, all of which add cost and time.

The neighbours

Look over the fence. Older buildings on shallow footings, hard on the boundary, need underpinning or stiffer shoring, and heritage buildings next door bring their own limits on vibration and movement. A neighbour's existing basement can help or hinder, depending on how deep it goes and what holds it up.

Shoring anchors also extend under neighbouring land, which needs the owners' agreement. If they refuse, the shoring gets heavier.

Contamination

A site with a past as a service station, a workshop or a factory can carry contaminated soil. That is an environmental consultant's work rather than ours, but it changes what every truckload of excavated material costs to remove, and a basement produces a lot of truckloads.

How we approach it

At feasibility stage we provide a desktop geotechnical assessment, a foundation strategy and a review of the authority assets and site constraints, then model the cost impact of each finding. We keep it to what a buyer needs to price the site, and say plainly where boreholes are still missing.

The cheapest time to find a problem in the ground is before you own it.

ACSES provides geotechnical and civil engineering advice from feasibility stage across Sydney. Talk to us about a project.

Building Relationships Beyond Structures