1. Builders Margin: 

A builders margin is added onto whatever is not specified in the tender package, this includes things like tiles where a rate has been given, say $50 per m². If you select a tile for $55 per m² then you will be paying that rate + the builders margin say 20% which means that you will be paying $61 per m² for the tiles. Builders margins range from 15-25% so it is worth checking the builders margin and finding one that is not too high. It is also worth specifying as much as you can before putting the project out to tender to keep these costs low. Any discoveries or changes that you make onsite will mean that you are paying for the cost of these plus the builder's margin. Say you want to change a wall or put a window in, you will pay for the cost of this plus the builders margin (15-25%).

2. Excavation Rates:

This is another area where costs can blow out of the budget as it is not generally included in the initial tender. The rate should be between $30-50 for soil and between $40-$150 per m³ for rock.

3. License and insurance:

Ask for the builder's contractor license number using the fairtrading checker. Also check for previous disputes. Builders require the following insurances so before engaging one check that they have these: Home building compensation cover, contract works insurance, professional indemnity insurance, public liability insurance, workers compensation insurance, get them to send you a copy of each. Here is a full list from fairtrading NSW.

4. Project Timeline:

Check what the project timeline is and if they have to recompense you if they go over it, this is important if delays cause you a cost such as paying rent, make sure it covers what a delay will cost you.

5. Allocated rain days:

Make sure they have enough allowance in their timeline for rain days, check what they consider to be a rained off day and check how many rain days on average happen during that period in a year and make sure they are covered in the project timeline.

6. Talk to a previous client:

Ask the builder to talk to a previous client and go and visit the project, this show that the builder is capable of doing a good job and that they can retain good relations with a client. Ask them for a list of projects to drive around and look at.

7. If they lie or mislead you in any way do not use them:

If at any stage they mislead you or lie in any way, or if you don't have a good feeling bout them, don't use them. You really need a builder you can trust.

Here is a checklist we use before engaging a builder:

Checklist for Hiring a Builder

List of Builders

Here are a list of builders on the Central Coast that are genuine that we've worked with in the past, do your own due diligence though, as it can depend on which foreman is allocated to a job. Have a talk to them and see if you are happy:

Smaller jobs:

Baxter Homes

All Solutions Building

Larger Jobs:

Red Eye Constructions

North Construction and Building

Stevens Construction

There is also an article on becoming an owner builder here.