Any vagueness in a construction service specification is bad, and a non-existent construction service specification is, in a way, maximally vague.
Honestly, I don’t see the added value of a construction service specification when building with an architect and individual contracting. You have conversations during the basic evaluation, align expectations, get shown the architect’s "standard" for specific equipment and the cost estimate, and that’s it. What added value does a construction service specification offer if you are not building turnkey or at a fixed price? I only see unnecessary work in the form of a highly precise construction service specification that becomes obsolete at the latest during the first workshop planning meeting. Since you pay for this unnecessary work, I prefer to forgo it and instead talk twice as much with the architect rather than invest time and energy in the construction service specification.
This might be different with the architect and building with a general contractor (GU), but with individual contracting? What is the point? Many positions shift so much in price due to the tenders that either cheaper or more expensive options can be used. In addition, things are added or dropped during the planning process. What good is it to have a construction service specification at the start that lists plastic windows, but due to client wishes, large window areas require wood for structural reasons? Or if the appropriate air-to-water heat pump (LWWP) is selected after the energy/heating load calculation, but another one was initially listed in the construction service specification, and so on and so forth…
With a general contractor or turnkey contract (BT), yes, mandatory and as precise as possible because of the fixed price. With an architect and individual contracting, the construction service specification is outdated the moment it is written and therefore unnecessary. That’s how I handle it...