The short answer
Usually nowhere: most two storey extensions are reached from the existing landing through a new doorway, so the house keeps its stairs. A new stair is only needed when the landing cannot connect, and then it must meet Approved Document K: a rise of no more than 220 mm, a going of at least 220 mm, a pitch of no more than 42 degrees and 2 m of headroom.
The staircase is the part of a two storey extension people forget to plan, because most of the time there is nothing to plan. The new upstairs room is reached from the landing you already have.
The usual answer: through the landing
On a rear extension, the landing or a back bedroom usually sits against the wall the extension joins. A new doorway through that wall leads into the new room. On a side extension, the same happens through the side wall. No new stair, no lost floor space, and the ground floor of the extension is left free for the kitchen or living room.
The thing to check is levels. The floor of the new room needs to meet the landing without a step, or with one that building control will accept. Measure your landing’s height above the ground floor early and give it to the designer. On houses with a half landing or a split level at the back, the new floor may line up with the half landing rather than the main one.
If the only way through is via an existing bedroom, you lose that bedroom as a private room, or you give up part of it to make a corridor. That is often the real design decision in a two storey extension, more than the stair itself.
When a new stair is needed
A new staircase makes sense when:
- the existing landing cannot reach the new room at all;
- the extension is large enough to work as a separate suite, such as an annexe-style bedroom and shower room;
- you want to move the main stair to free up the old hall.
A new stair inside the extension takes up floor on both levels, roughly the size of a single bed on each floor for a straight flight. On a small extension that can use up most of the gain.
The building regulation limits
In England, stairs in a house must meet Approved Document K. For a private stair:
- Rise (the height of each step): no more than 220 mm.
- Going (the depth of each tread, measured nose to nose): at least 220 mm.
- Pitch: no steeper than 42 degrees.
- Twice the rise plus the going should fall between 550 mm and 700 mm, which keeps the step comfortable.
- Headroom: at least 2 m above the pitch line.
- Handrail: on at least one side if the stair is under 1 m wide, on both sides if wider.
- Guarding: at least 900 mm high on stairs and landings, with no gaps a 100 mm sphere could pass through.
The stair also forms part of the escape route in a fire, which affects where it can come down. If the bedrooms upstairs rely on the stair rather than escape windows, it may need to be a protected stairway leading to an outside door.
Stairs in a prefab extension
Factory-made panels and modules are usually built with any stair opening already formed in the upper floor. That makes the decision a fixed one: the stair position must be settled on the drawings before the floor is made. Some module makers deliver the stair in place; with panel systems the stair is usually fitted on site once the shell is weathertight.
Before you decide
Draw both floors with furniture in them. A doorway from the landing that costs you a few square metres of an existing bedroom often gives a better house than a new stair that takes space on both floors of the extension. If you are still weighing the options for a two storey prefab extension, ask firms to show you both layouts.