During this last session of Passive House Training I learned that 25-35kWh per square meter and year are acceptable, and respectable space-conditioning energy numbers for a remodeled building. At the same time, any other requirements like air-tightness do not differ from the original Passive House requirements. While a remodel that achieves 25-35 kWh per square meter and year does not pass PHPP (Passive House Planning Package) calculations, it is a substantial achievement and definitely a significant improvement over performance before the upgrade (potentially up to 80% and more energy saved).
The Passive House Institute has researched and shown, that there is tremendous value in retrofitting to Passive House standard. Adding Passive House insulation and achieving high air-tightness helps avoid condensation and due-points at the thermal bridges and inside wall assemblies, therefore protecting the existing structure far better than a lesser improvement. This also ensures healthier indoor environmental quality.