What makes a Victorian terrace worth restoring?

Start with the street
A terrace is only as strong as its row. We look at the whole street before the individual house: are neighbouring roofs sound, are frontages maintained, is the row mostly owner-occupied or let? A well-kept street of period terraces in Nottingham or Derby holds its value through cycles in a way that a lone restored house on a neglected row cannot.
Location within the street matters too. End-of-terrace houses gain a side window run; mid-terrace houses depend entirely on front and rear light, which shapes what a reconfiguration can achieve.
The fabric checklist
The questions that decide a purchase are structural, not decorative. Is the roof covering original slate or a later concrete tile that has overloaded the timbers? Are the walls solid brick in lime mortar — which breathes and can be repaired — or has someone smothered them in cement render that traps moisture? Are the chimney breasts intact, and do the original floor structures run front-to-back, which makes replanning simpler?
Cosmetic decay is a buying opportunity. Structural compromise is a cost. The skill is telling them apart before exchange, not after.
What survives inside
The houses we most want are the ones where the original joinery survived the twentieth century: panelled doors, skirtings, architraves, stair balustrades, ceiling roses, cornicing, and — increasingly rare — original sash or casement windows. Each surviving element is one we can repair rather than reproduce, and repair is always better than replica.
Paradoxically, the unmodernised house is often the best preserved. The owner who never had the money to 'improve' the house also never had the money to rip out its character.
When we walk away
We pass on houses where the numbers only work by shoehorning — squeezing a third bedroom into a box room, or a bathroom into a cupboard. If a terrace cannot take a generous layout, no amount of finish will make it a good home, and we would rather leave it for a buyer whose plans fit the house.
Sloane & Sable restores character British homes across Nottingham, Derby and the wider East Midlands. If you own — or are selling — a property that fits this journal's subject, we would like to hear about it.