Resale value - whether a renovation or of the whole house - is not a priority for many Swiss homeowners, as many Swiss homeowners only leave their houses feet first.
The following is my opinion, based on studying the market in my area, and having toyed with selling for the last ca.10 years. As always, others may think differently...
The only things you can count on to increase value:
If you are able to increase the (legal) m2 living space.
If you have Ausnutzungsreserve allowing the next buyer to increase living space.
General rise in the price of land - but only if your house type/size can keep up with the price, or be renovated/torn down to build to match rising land prices.
The problem with interior renovation is that tastes change quickly - what was done 5 or 10 years ago, although still perfectly serviceable, might not be what a buyer will want today, and even sometimes becomes a liability if the buyer wants to take it out.
We bought 11 years ago. Renovated top to bottom - floors, walls, kitchen, bathrooms, added a bathroom, woodwork, wiring, re-did the facade and roof, new balconies, put in a patio and terrace. Last year we put in a new furnace. Spent ca. 35-40% of the purchase price again. And then there's the garden...
We do not expect to get a penny of that back when we sell, because it was (furnace excepted) done 11 years ago. That's too long ago to be 'im Trend' or even of value anymore.
The things that hold their value - the roof, facade, wiring, plumbing, heating system - are sort of 'invisible' in that a buyer expects such things to be in good working order, only really notices (and low balls your price) when they are not.
Bottom line - IMO, renovate for yourself, for your enjoyment.
If you are only here for the short term and have to consider resale, go Swiss minimalist white.