Cost guide
Two storey extension cost and what makes it different
A two storey extension is not two single storey extensions stacked. It buys the second floor for a much smaller uplift than the first, because you dig once, roof once and scaffold once. That is the single most useful thing to understand before you set a budget.
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The economics, honestly
The most expensive elements of any extension are the ones you only pay for once. Foundations, drainage, the roof covering, the scaffold and the site setup all serve both floors. Add a first floor and you are largely paying for walls, a floor structure, windows and internal fit out, so the cost per square metre falls sharply.
That is why a two storey extension often looks poor value on the headline number and excellent value on the arithmetic. If you need a bedroom as well as a bigger kitchen, doing both in one build is far cheaper than doing them in two.
Costs that only appear on two storey work
- Deeper or wider foundations, because the load carried down is roughly doubled
- Full height scaffolding for longer, with edge protection and often a temporary roof
- Tying the new roof into the existing roof rather than simply abutting a wall
- Matching the existing brick and tile at high level where it is far more visible
- Reworking the upstairs landing so the new rooms are reached sensibly
- More likelihood of a party wall award if you build near a shared boundary
The upstairs decisions that set your final figure
- Step 1
How the new rooms are reached
If the existing landing cannot serve them without walking through a bedroom, the layout change is part of the project rather than an afterthought.
- Step 2
Whether an ensuite goes in
New drainage at first floor level, ventilation and tiling. Cheaper when stacked above an existing wet area.
- Step 3
Window rhythm on the front elevation
Where the extension is visible from the road, matching proportion, cill height and materials matters to the planners and to the finished look.
- Step 4
What happens beneath
Most people take the chance to open up the ground floor at the same time. It is far cheaper while the steels and the scaffold are already there.
One team, one written price
We run the design, the structure and the build with our own team, so the ground floor and first floor are the same contract and the same accountability. Free site visit, fixed price written quote, 5 year workmanship guarantee.
FAQs
Common questions
Is a two storey extension roughly double the price of a single storey one?
No, generally well under double. The foundations, roof and scaffolding are shared, so the second floor is the cheapest floor area in the project.
Will a two storey extension need planning permission?
More often than a single storey one, particularly at the side or where it comes forward of the building line. Some rear two storey work is permitted development within strict limits, so check with your local planning authority or the Planning Portal.
Do I need a party wall agreement?
If you excavate near or build up to a shared boundary, usually yes. It is a process rather than a problem, but allow time for it in the programme.
Can we live in the house while it is built?
Usually yes until the break through, which we plan and agree with you. Upstairs work is noisier, so we agree working hours before we start.
How difficult is it to match the existing brickwork?
On a two storey elevation it matters, so we source the closest match and set out the courses to line through with existing openings. Weathered brick never matches perfectly, and we say so before we start rather than after.
Should the ground floor be reworked at the same time?
If you have been considering it, yes. Doing it later means bringing steels, dust and disruption back into a finished house.
Ready to price your two storey extension cost and what makes it different in Kent?
Free site visit, fixed-price written quote and a 5 year workmanship guarantee — backed by 40+ years on the tools and our workshop in Biddenden.
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