The figures start as a worked example. Replace them with yours.
The floor area you will sell.
The total floor area you will build, including walls and shared space.
Architects, engineers, and other consultants, as a share of the build cost.
An allowance for cost overruns, as a share of the build cost.
Agents, marketing, and legal costs of selling, as a share of the sales value.
The profit you need, as a share of the sales value.
Interest on borrowed money. Building costs are drawn evenly, so on average half is borrowed; the land is borrowed for the whole build period.
Taxes and fees on buying the land, as a share of the land price. Enter the rate that applies to you.