There a number of things to consider when first starting out on a project:
Budget:
Allocate a budget specifically for the stair/balustrade in your build project, prices vary enormously depending on your taste and requirements, from the cheapest timber stairs at around £800-£1000 a flight to a single-story bespoke staircase and balustrade, starting at around £20,000
Time:
A bespoke staircase should be considered at the very start of a build project, it takes a long time to design and produce a bespoke flight, there are many stages; CAD drawing, steel substructure, steel super-structure, treads, balustrade, handrail, installation and finishing.
There is more often than not a requirement to add extra steel to support a bespoke stair or at least make additions to the structure of a building to support the weight or forces of a cantilevered or curving staircase, additionally, access to install a large heavy object is often easier in the early stages of a build in the long run saving time and money for you the customer.
Space:
There are many nuances to do with staircase design that are affected by available space, a staircase is essentially a means of getting from one level to another but user experience should always be considered during the design phase of a bespoke staircase.
Often staircases are put in the smallest amount of space required but this can lead to an unergonomic use of stairs, requiring the user to travel in unpleasing ways around the space.
With more space a bottom of a staircase for example the bottom steps can be splayed out and curved to greet users from many angles and directions.
With more space, sharp landing corners can be spread into gradual winding corners to make step feel more consistent and eliminate jarring changes of direction.
The shape of a staircase directly impacts the shape and consequently the feel of the handrail, if the staircase takes a sharp corner, the handrail will have to follow suit, so the feel of the rail under your hand is something that has to be considered right at the very beginning of the design process.