Direct answer: the existing opening is the key
In older homes, the opening, wall condition, trims and ventilation can be as important as the new window itself.
Irregular openings
Measure in several points and expect finishing work. Uneven openings can affect airtightness and appearance.
Preserving character
Colour, profile shape, divisions and opening type should respect the existing façade and interior style.
Ventilation and condensation
More airtight windows can change ventilation habits. Condensation may appear if indoor humidity is not controlled.
Common mistakes
Ignoring the opening condition.
Forgetting ventilation.
Choosing a style that clashes with the façade.
Leaving shutters and trims undefined.
Practical recommendation
Ask for a complete assessment: window, glass, shutter, opening, finishing and ventilation.