The principle: one good route beats scattered shelves
Cats do not want random high spots; they want a route — a connected path that climbs, crosses, and ends somewhere worth being. In a small flat that is good news, because a single well-planned route along one wall delivers more territory than furniture scattered through rooms you cannot spare. Design the route from your cat's point of view: up in easy stages, along a high run, and finishing at a destination perch with a view, ideally out of a window. Every stage should be reachable in one comfortable hop from the last, which for most adult cats means modest vertical gaps and landings deep enough to turn around on.
Fixings and dimensions that keep a cat safe
A shelf a cat lands on takes far more force than its static weight, so fix into wall studs or use proper solid-wall fixings rated well above your cat's weight, and test each shelf with your own hard downward pull before the cat ever uses it. Shelves should be deep enough for a full sprawl on the main perch — most cats want to lie, not just sit, at the top — and surfaced with something grippy: carpet offcut, sisal matting, or a washable cushion fixed so it cannot slide off under a landing cat. Avoid a route above your bed or your desk chair unless you enjoy a cat dropping in, literally, and keep the top perch away from ceiling light fittings and picture frames the route will dislodge.
A workable layout for a typical one-room or one-bed flat
Start the climb from furniture you already own: sofa arm to a low shelf, shelf to a higher shelf staggered sideways so the cat climbs in a zigzag, then a long high shelf running to the window, ending in a wide window perch with a cushion. Under the route, place one tall scratching post — tall enough for a full upright stretch — that doubles as a climbing column and a step in the route. Clear one level of an existing bookshelf, line it, and make it a mid-height resting stop. That arrangement adds three or four resting heights and a full circuit while taking up no new floor space beyond the scratching post, which is the honest currency of a small flat.
Fitting the route to your particular cat
A kitten or an agile young cat will use tighter spacing and higher finishes; a senior, a heavy breed, or a cat with stiff joints needs lower stages, ramps or steps instead of hops, and a top perch it can reach without a leap of faith. Watch the first week closely: the stage your cat hesitates at is too big a gap, and the shelf nobody uses is usually in a draught, in full sun, or facing a wall instead of the room's life. Move one thing at a time and let use, not the plan, settle the design. Keep dangling cords, trailing plants, and fragile ornaments off the route entirely, since a climbing cat will investigate everything at its new eye level.
Planning it once, properly
Measuring, spacing, and fixing order are easier to get right on paper than halfway up a ladder with a drill in hand, especially when you are tailoring the route to one cat's age and reach. If a spacing or fixing question comes up while you plan, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.