Is a two storey extension twice the price?

Rarely. The shared elements mean the second floor typically adds proportionally less, unless the existing foundations need strengthening to carry it.

Is it harder to get planning permission for two storeys?

It attracts more scrutiny, because the effect on neighbours and the street is greater. Whether it is harder depends entirely on the plot and the local policy context, which the local planning authority can confirm for your address.

Will the existing foundations take it?

Sometimes yes and sometimes no. It depends on their depth and the ground beneath them, which is why a trial hole is worth digging before the budget is fixed.

Can you handle the structural side too?

Yes, we coordinate structural calculations alongside the drawings so the beam on the calculation sheet is the beam on the drawing.

Cost guide · Cornwall

Two storey extension costs in Cornwall, and why the upper floor is the cheaper half

The instinct is to double the single storey figure. In practice the expensive elements are shared between the two floors, so the second storey usually adds proportionally less than the first. What it does add is planning risk, structural load and a more involved connection into the existing house, and each of those has a cost attached.

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Shared costs and added costs

Foundations, roof, scaffolding, drainage and the site setup serve both floors. The added costs sit in the first floor structure, the extra external wall area, additional windows and, frequently, a heavier foundation to carry the extra load.

Shared: foundations footprint, roof, scaffold, drainage connection, site establishment
Added: intermediate floor structure, upper external walls and finishes, additional glazing
Sometimes added: foundation widening or underpinning where the existing foundation is shallow

Planning is the bigger variable

A first floor addition changes the street elevation and affects neighbour outlook, so it attracts a different level of scrutiny to a rear single storey. Conservation Areas, AONB settings and Article 4 directions all narrow what will be accepted. Establish the planning route before spending on a design that could not have been consented.

Structure and building control

Two storey work almost always needs structural calculations, for the new floor, any openings formed into the existing house and the foundation design. We coordinate calculations onto the drawings so building control receives one consistent package rather than two documents that disagree with each other.

Living through it

There is normally a period around the roof tie-in and the structural openings when the house is genuinely disrupted. We plan that sequence deliberately and tell you which weeks are difficult, rather than discovering it together.

Frequently asked

Straight answers.

Is a two storey extension twice the price?: Rarely. The shared elements mean the second floor typically adds proportionally less, unless the existing foundations need strengthening to carry it.
Is it harder to get planning permission for two storeys?: It attracts more scrutiny, because the effect on neighbours and the street is greater. Whether it is harder depends entirely on the plot and the local policy context, which the local planning authority can confirm for your address.
Will the existing foundations take it?: Sometimes yes and sometimes no. It depends on their depth and the ground beneath them, which is why a trial hole is worth digging before the budget is fixed.
Can you handle the structural side too?: Yes, we coordinate structural calculations alongside the drawings so the beam on the calculation sheet is the beam on the drawing.

Considering a two storey extension?

We will assess the planning route and the structural implications together, at the free first consultation.

Free · No obligation

Book your free consultation

Tell us about your project — we usually reply within 2 hours (working day).