The staircase is the single biggest constraint on most loft conversions. It has to arrive somewhere with headroom, leave the floor below still working, and comply with rules on pitch, going, rise and headroom that are not negotiable. Many designs are decided by where the stair can go rather than by what the owner wanted upstairs.
What the rules actually constrain
A staircase to a loft is a normal domestic stair and has to meet the same requirements: a maximum pitch, consistent rise and going, handrails, and headroom measured vertically above the pitch line. Approved Document K allows a reduced headroom in the specific case of a loft conversion where the full dimension cannot be achieved, measured at the centre and at the edge of the stair, which is the concession that makes many conversions possible at all. It is a defined allowance rather than a general relaxation.
Where it normally goes, including small terraced house loft conversion stairs
Directly over the existing staircase is the usual answer, because it reuses the vertical space that is already circulation and it lands where the headroom under the ridge is best. Where that will not work, the common alternatives are taking space from a bedroom below, running the stair along a landing wall, or on a terrace putting it over the return. Each takes floor from the storey below, and that loss is real: a conversion that turns a three bedroom house into a three bedroom house with a loft room has not gained what the owner expected.
Space saving loft conversion stairs, and what is allowed
Alternating tread stairs, where each tread is cut away on alternate sides, are permitted in tight situations and only for access to a single room, not as the main stair to a storey with more than one habitable room. Spiral stairs are allowed and have their own geometry rules, and they are hard to carry furniture up and awkward as an escape route. A loft ladder is not a staircase and does not make the space a habitable room. If somebody proposes any of these, ask which rooms the loft will have and whether the arrangement is compliant for that number.
Fire escape decides more than you expect
The stair is not only circulation, it is the escape route, and adding a storey means the route from the new floor to the final exit normally has to be protected: fire doors onto the stair at every level, a stair enclosure, and mains linked alarms. That is why an open plan ground floor complicates a loft conversion considerably, because the protected route cannot pass through an open kitchen without an alternative approach such as a sprinkler system. Establish that early, because it can change the whole project.
Other questions people ask
Can I use a spiral staircase for loft conversion access?
Yes, and it has its own geometry rules, is awkward for furniture and is a poor escape route. It is usually a last resort rather than a first choice.
Are alternating tread stairs allowed?
In tight situations and only for access to a single room, not as the main stair to a floor with more than one habitable room. Check what the loft is intended to contain.
Does the loft stair have to go above the existing one?
No, and it usually does because that is where the circulation and the headroom already are. Anywhere else takes floor space from the storey below.
Why does my open plan ground floor matter?
Because the escape route from the new storey normally has to be protected to a final exit, and it cannot pass through an open plan kitchen without an alternative approach such as sprinklers. It is one of the first things to establish.