The Single Storey Extension: Five Design Choices That Make or Break It

A single storey extension lives or dies on five decisions, and none of them is the colour of the units or the tiles. They're the choices made on the drawing board, long before anything is built, and they're what separate an extension you love from one that merely adds square metres.

Get these five right and the room works. Get them wrong and no finish will rescue it.

A well-designed single story extension rests on these decisions, so here they are, in order of importance.

Choice one: flat roof or pitched

The roof is the first and biggest decision, because it shapes the light, the ceiling and even the planning route. A flat roof is cheaper, keeps the height down, and crucially allows rooflights and lanterns that a pitched roof can't. A pitched roof costs more, around £4,000 to £7,000 extra, but gives a loftier, sometimes vaulted ceiling and suits period houses.

There's a planning dimension too. Under permitted development, a flat roof is effectively limited to 3 metres in height, while a pitched roof can reach 4 metres, provided the ridge sits below the existing house.

For most single storey rear extensions, a flat roof with large rooflights is the popular modern choice. For a period home or a conservation area, a pitched roof that matches the house often wins. A mono-pitch, or lean-to, sits between the two and works well on side returns.

Choice two: how you bring in the light

This is the choice that most transforms how the finished room feels, and the one people most often skimp on. Push out the back and the centre of a deep extension can turn dark, so the glazing strategy is everything.

The toolkit is simple: wide bifold or sliding doors onto the garden bring light in low, and rooflights or a roof lantern drop daylight straight into the middle of the plan where wall windows can't reach. A single well-placed lantern can rescue a space that would otherwise feel gloomy beyond three or four metres from the rear wall.

Rooflights run roughly £900 to £2,500 installed, and a roof lantern around £2,250 upward. It's money that buys the single biggest improvement in how the room feels, so it's rarely the place to economise.

Choice three: the floor level transition

Here's the detail nobody thinks about until it's a problem. On older houses, the existing ground floor, often a suspended timber floor, sits at a different level to the new extension's slab.

If that level change isn't resolved at the design stage, you end up with an awkward step between old and new, exactly where you want seamless flow. The fix is a flush, level threshold throughout, designed in from the start so the finished floors run continuously from the original house into the extension and out to the garden.

It sounds minor, but a trip step in the middle of an open-plan kitchen-diner undermines the whole point of the extension. Getting it flat is a mark of a properly designed scheme.

Choice four: how the new space connects to the old

An extension isn't a separate box, it's a new part of the house, and how it joins the existing rooms decides whether the whole ground floor works better or just gets bigger. This is where the structural opening comes in.

The decision is how far to open up the original rear wall: a modest doorway, or a full-width structural opening that makes the old and new one continuous space. The wider the opening, the more seamless the flow, but the bigger the steel beam required to carry the load above.

This matters most on a kitchen extension, where the aim is usually a single open kitchen-dining-living space. Taking the opening to full ceiling height, rather than leaving a downstand beam, is what makes old and new read as one room rather than two knocked together.

Choice five: open plan or broken plan

The final choice is about how the space is divided, and the trend is shifting. Fully open-plan has dominated for a decade, but broken-plan is gaining ground.

Open-plan gives one big sociable space. Broken-plan uses subtle dividers, a change in floor level, a part-wall, a glazed screen, sliding doors, to define a kitchen, dining and living zone while keeping the light and connection. It tames the noise, cooking smells and lack of privacy that fully open rooms can suffer.

Neither is right or wrong. The choice is about how your household actually lives, whether you want one unbroken space or gently zoned areas, and it's far easier to design in now than to retrofit later.

Five choices, one well-designed room

The finishes get all the attention, but these five choices are what actually determine whether a single storey extension succeeds: the roof, the light, the floor transition, the connection to the house, and the open-versus-broken plan. Settle them thoughtfully on the drawing board and the room delivers everything you hoped for. Extension Architecture, a RIBA-chartered London and Surrey practice with in-house structural engineering, designs single storey extensions around exactly these decisions, because a beautiful finish on a poorly resolved layout is money spent in the wrong place, while five good choices make a room that works for decades.