There’s 2 options to consider when building a new home, a custom designed home or a project home. Generally the guidance will be that with a custom build it will be more expensive than a project build and while this is true for many cases you can build a custom home for similar or less.
Larger builders will have a number of floorplans to choose from, each varying in many different ways. They will have different facades for each of their designs and inclusions normally come in standard and upgraded ones. If you choose a package they wont normally credit any items in them so its a take it or leave it.
They will have a base price which is for the build itself and excludes any other variations for site conditions and location. Dont just compare the base price as some include expensive upgrades in their base while others dont to make the price more attractive. These are normally inclusions that most people will upgrade but will depend on your budget.
Most project builders will allow significant changes to their plans including exterior/interior walls, moving bedroom/bathroom/alfresco locations and many other changes. Normally everything except for the staircase can be changed.
Some volume/project builders have plans that are cheaper but you cant make any modifications to it.
A custom build is one where you have plans drawn up by an architect or draftsman and this is given to the builder.
Its recommended to do the concept design first before doing any applications for CDC/DA as the cost of building your design could be a lot more expensive than you anticipated. Most builders I found will want to do the work afterwards because they can design it in a way they they are familiar with and I feel they will do a more efficient build than an architect would as they dont know what things cost.
Pricing is not based on a sqm rate as there are many factors that go into the cost. You can however ask the builder for an idea of what homes they typically build and what their price p/sqm is because if you need to build something for say $1800 p/sqm but they have never built for under $2200 p/sqm then theres no point wasting yours or their time. Not all will give you this however.
Its a bit of a misconception to say a custom build will be 20-50% more than a project home. Most smaller builders will likely be in this range however as they dont have the buying power or efficiencies that comes about spreading costs across multiple homes.
You can check this out if you want to get more details about Choosing a Builder
There are a number of project/volume builders who can build custom homes too incl. Vogue Homes, Fairmont Homes, Fowler Homes and a number of others. When I did a tender off each of them the price varied considerably, by up to 170k.
Generally a builder with just a few employed staff who would build a handful of properties a year. In some areas they can be price competitive but I found they were considerably more expensive.