How the finish level changes a building price.

Every home has two kinds of cost. There is the part you cannot see once it is finished: the footings and slab, the frame, the roof structure, the plumbing and drainage in the walls, the wiring, the insulation. And there is the part you can: the flooring, the wall and ceiling finishes, the tiles, the kitchen fronts and benchtops, the tapware and sanitaryware, the doors and their hardware, the lighting, the windows you look through. A finish level is a decision about the second part only.

On Modern GBQ the levels are Economical, Standard, Good and Advanced. Choosing one applies that level's default product to every line in the estimate that needs a product: the plainest listed product that does the job at Economical, the mid-range most homes are built with at Standard, a step up in material and brand at Good, and premium products at Advanced, along with the extra labour some of them need, such as large-format tiles or stone benchtops. The level is a shortcut for hundreds of product decisions, made consistently, in one click.

What it changes is therefore wide but shallow. It touches almost every room, because almost every room has a floor, walls and a door. It is felt most in the rooms that are full of products: the kitchen, where the fronts, the benchtop, the tapware and the appliances all move together, and the bathrooms, where the tiles, the tapware and the fixtures do. In a bedroom the difference between levels is mostly the floor covering and the door hardware.

What it never changes is the structure and the services. A Good kitchen and an Economical kitchen need the same slab under them, the same frame around them and the same drainage beneath the sink. The site works, the footings, the roof structure and the rough-in are decided by the block and the design, not by the finish. This is why a change of level moves the total less than people expect on a large home, where the structure is a large share of the cost, and more than they expect on a kitchen or bathroom renovation, where the finishes are most of what is being bought.

The useful way to work with the level is to set a base and move only the rooms that matter. A Standard base with a Good kitchen and a Good main bathroom is a common and sensible answer; so is an Economical base for a rental with the difference spent on better glazing and insulation that lower the running cost. Every default can be swapped for a listed alternative, with a photo to judge it by and a short reason recorded, and the estimate re-priced. Each re-price is a new numbered version, so the Good and the Advanced benchtop can sit side by side, line by line, rather than being argued about from memory.

Two cautions. First, choose the level and the products in the estimate, before a builder prices, because a swap after the quote is a variation and a re-quote. Second, decide where a level change stops: a finish well above the rooms next to it looks odd at the doorway, and the temptation to refresh those rooms too is how a kitchen becomes a renovation.

The finish level is the easiest number in a budget to move. Knowing exactly what it moves is what makes it a decision rather than a surprise.