How do I bathroom / kitchen / extension? etc.

the entire gaff

they had a loft conversion and removed it to get closer to the standard
https://www.studiobarc.org/work/strathbungo-ecorefurb

Innovative Design
A key project driver for the client is the reduction
of energy consumption and reference will be made to Passivhaus
practice. Although the full Passivhaus standard cannot be achieved in
an older property of this kind, the scope of works will include
upgrading of all windows, external insulation to side and rear walls,
reduction of air leakage, introduction of energy efficient mechanical
ventilation with heat recovery. The extension will be designed to
achieve very good levels of thermal insulation and airtightness. The
large southern facing windows will contribute to reduced energy use
through passive solar gain.