A necessary step of work has not yet been fully costed. For the property purchase you nonetheless want to know whether your budget is roughly adequate. Instead of entering an apparently precise average, you can present clearly labelled variants side by side. The range must be based on professionally delimited information, not on an arbitrary safety percentage.
First clarify what the span covers
Have it described which service is included in both variants and which uncertainty causes the difference. If technical findings are still missing, these remain visible as an outstanding task. A cost range does not automatically answer the question of whether the work is feasible at the assumed scope.
Record the source, date and binding nature for the amounts. A rough conversational figure does not become a fixed price by being entered into a table. Contractual consequences of an offer or a later change will be examined separately.
The fictitious money comparison
After the other purchase uses already recorded in the case, 28.000 Euro remain. For the work here expressly only assumed, two variants of 12.000 and 20.000 Euro are used. Afterwards, 16.000 or 8,000 Euro would respectively remain available.
An entered average of 16.000 Euro would show a remaining amount of 12.000 Euro. But it is not proof that the actual payment will lie exactly in this middle. The two edge cases make the personal consequence clearer.
Specify your own limit concretely
Describe which other actual purposes are to be financed from the remaining money. If 8,000 Euro is not sufficient for this under your assumptions, the existing purchase planning depends on further clarification. The article does not name any generally sufficient reserve.
If both variants appear bearable, the technical and contractual uncertainty is still not resolved by that. The financing perspective and the technical execution question are brought together, but not mutually replaced.
Do not treat several uncertainties independently and unnoticed
If there is additionally an open financing cost item or an unclear move-in date, also show a jointly less favourable course. Do not assert any probability for this. The point is whether the same free funds are meant to cover several simultaneously possible burdens.
The decision becomes comprehensible if you can say which scope is already clarified, which variants were calculated and which information is still missing before a commitment. A nicely smoothed total sum is less helpful than this openly stated limit.