Selling property, what costs to expect?

Is only the built-up part counted or the entire plot?

As a distribution key for development costs, I only know the proportional plot size in relation to the "overall area" of the development measure. Whether a plot is built on at all or to what extent does not matter in this regard.

Is there a rough guideline for assessing development costs?

No. These costs are too different for that. There are fundamental differences regarding the share of the residents at all: their contribution is significantly lower for a federal road when looking at the overall measure than if they are the sole beneficiaries of a residential street.