We are asked this often, and we do not always answer in favour of building. Some houses can be transformed; some have a ceiling set by the plot, the layout or the street, and knowing which you have is worth more than a quote.
What the plot allows
Garden depth, boundary distances, access for machinery and the position of drains set the realistic limit on what can be added. That limit is fixed regardless of budget.
What the layout allows
Stair position, structural walls and ceiling heights determine how much a plan can genuinely be reorganised. Some layouts open up beautifully; others fight every change.
Disruption and living arrangements
A major refurbishment is months of living around a building site or renting elsewhere. That is a real cost, and it belongs in the comparison.
Overdevelopment
There is a point on any street where further spend stops being reflected in value. We will say so if we think a project is heading there.