Extensions
Single storey or double storey extension — which is better value
A second storey is rarely double the price of the first, because the expensive parts are already paid for. That changes the maths more than people expect.
The instinct is that two storeys costs twice as much as one. It usually does not, and understanding why changes how the decision looks.
The expensive parts are the top and the bottom
On a single storey extension, the two most expensive elements per square metre are the foundations and the roof. Groundwork means excavation, muck away, concrete, drainage and a slab. The roof means structure, covering, insulation, guttering and the junction with the existing house — which is the fiddly, expensive part.
Adding a second storey does not add a second set of either. You dig the same hole and you build the same roof. What you add is walls, a floor structure, more windows, more internal finishing, and scaffolding.
The practical consequence: the second storey typically costs meaningfully less per square metre than the first. If you are confident you will want the space eventually, building it now is almost always cheaper than building it later, when you would be paying for a second set of foundations, a second roof, and a second round of disruption.
There is a caveat. The foundations for a two storey extension carry more load and may need to be deeper or wider than a single storey would have, and the existing structure has to be checked for its ability to take the extra. That is a structural engineer’s question and it is site-specific.
Where the single storey wins
Planning. This is the big one. Single storey rear extensions often fall under permitted development, within limits. Two storey extensions have tighter permitted development limits — including a requirement to sit further from the rear boundary — and are far more likely to need a full planning application. That is a statutory eight week determination period, a fee, and a real chance of refusal or conditions.
Two storey extensions also attract more neighbour objection, because they overlook and they overshadow. A single storey extension is rarely controversial; a two storey one at the back of a terrace frequently is.
Disruption. A single storey rear extension can often be built largely outside the existing house, with the through-opening left until late. A second storey involves work at first floor level, usually through a bedroom, with scaffolding up for weeks and a period where the existing roof is opened.
Time. Twelve to sixteen weeks against sixteen to twenty-four, before you count the extra planning time at the front.
Party wall. More likely to be engaged, and engaged more seriously, on a two storey job — particularly on terraces and semis where you are building up against a shared wall.
Where the double storey wins
Bedrooms are what change a house’s value band. An extra reception room adds useful space. An extra bedroom moves the house into a different search filter, and that is a step change rather than an increment. If the goal is to avoid moving, bedrooms are usually what the family actually needs.
Cost per square metre. As above — the second storey is cheaper than the first per unit of floor area.
One period of disruption instead of two. Building it later means starting again: skips, scaffolding, dust, and a house full of trades for another four months.
What the choice usually comes down to
Not cost per square metre, in practice. It comes down to three things:
- Do you need bedrooms or living space? If it is bedrooms, single storey does not solve the problem and you should be comparing double storey against a loft conversion, not against a single storey.
- Is the planning risk acceptable? In a conservation area, on a corner plot, or where a two storey extension would overlook a neighbour’s garden at close range, the answer may be no regardless of budget.
- Will you actually still be here? If you are likely to move in three years, the disruption of the bigger job may not repay itself, and the value uplift on an extension rarely matches its cost in the short term.
The third option people skip
If what you need is a bedroom, compare both against a loft conversion before deciding.
A loft conversion uses a roof space you have already paid for. It does not need foundations, it does not take garden, and on many houses it falls under permitted development where a two storey rear extension would not. The constraint is head height — you need enough of it under the ridge, and the required floor build-up and new structure eat into what is there. A rough rule is that you want somewhere around 2.2 to 2.4 metres from the existing ceiling joists to the ridge before conversion looks straightforward, but it depends entirely on the roof structure. Traditional cut roofs convert more easily than modern trussed ones, which need substantial structural work.
Worth an hour with a tape measure before you commit to either extension.
And the option nobody suggests
Sometimes the answer is not to extend at all, but to reconfigure. Houses of a certain age are full of space doing very little — a separate dining room nobody eats in, an oversized hallway, a downstairs layout that made sense in 1935. Moving a stud wall and reworking the circulation is a fraction of the cost of an extension and occasionally delivers most of the benefit.
A builder who tells you that is not talking themselves out of work. They are telling you where the money is best spent, which is worth knowing before you spend it.
What to do next
If you are weighing these up, the useful next step is a visit rather than a quote — someone looking at the actual house, the actual roof structure and the actual plot, and telling you which of the options is genuinely open to you.
Ask us to come and look. We would rather tell you early that something will not work than find out halfway through.
Planning work in Horsham?
Tell us what you have in mind and we will give you an honest view.
