Start with the construction
Peeling paint is visible; movement, failed joints and trapped moisture matter more. A useful assessment looks at cills, lower rails, glazing rebates, joints and the way water leaves the opening.
Keep the valuable fabric
Original proportions and sound material are worth retaining. Local repair can be the most respectful answer when it restores performance without dismantling what still works.
Replace with purpose
Where deterioration is extensive, a new window should reproduce the architectural logic—not just fit the hole. Sightlines, mouldings, opening method, glass and hardware all shape the result.