What Actually Drives the Cost of a Renovation

Removing a load-bearing wall means a steel beam sized by a structural engineer, padstones to carry it, temporary propping while it goes in, and making good afterwards. Removing a non-load-bearing partition means a skip. The two look identical on a floor plan and are not remotely comparable in cost.

Foundation work behaves the same way. An extension on firm ground with shallow foundations is straightforward; the same extension near mature trees, over a drain run, or next to a neighbour’s shallow footing may need deeper foundations, a drain diversion, or underpinning. None of that is visible when the quote is written unless someone has looked properly.

In older London property the electrics and pipework are frequently the real cost. A house that still has original wiring, or a mix of three previous owners’ partial rewires, needs a full rewire rather than an addition — and a rewire means chasing walls, lifting floors and replastering, which is why it costs a multiple of what people expect from the price of the cable.

The same applies to pipework, damp coursing and roof timbers. A strip-out often reveals work that has to be done before anything cosmetic can proceed. A quotation written without opening anything up is a quotation with an assumption in it, and that assumption is where budgets fail.

A detached house with a driveway takes deliveries onto site and stores materials. A mid-terrace on a controlled-parking street takes deliveries onto the pavement, moves everything through the house by hand, and pays for suspended bays and permits. The work is the same; the labour to support it is not.

Waste follows the same pattern. Strip-out generates a surprising volume, and where a skip cannot be placed outside the property it has to be handled differently and more expensively.

Once structure and services are resolved, the remaining variable is specification, and this is the part genuinely within your control. Large-format slim-framed glazing costs a multiple of standard doors in the same opening. Natural stone costs more than porcelain to buy, more to lay, and more to look after. Bespoke joinery costs more than good carcassing with a bespoke front.

The useful way to approach it is to decide where the money shows. Spending on the structural opening and the glazing changes how the whole ground floor feels; spending on a tap does not. A staged quotation makes this visible, which is why work is better quoted stage by stage than as a single figure.