In a duplex, the staircase is the spine
A duplex renovation (複式裝修), one home over two floors joined by an internal staircase, flows or fails on a single element most people treat as an afterthought: the stair. The staircase is the spine of the whole design. Get its position and generosity right and the two floors read as one home; get it wrong and you’ve built two flats that happen to be stacked. Around that spine sit three questions that separate a duplex from an ordinary flat: how to zone across levels, where the doubled services run, and what happens structurally when you open the space up. Here is how to think about each.
Public below, private above
The arrangement that consistently works is public functions below, private functions above. The lower floor, where you and your guests arrive, carries the living room, dining and kitchen: the social, active, come-and-go half of the home. The upper floor carries the bedrooms and study: the quiet, private half, buffered from the front door by a whole storey. This vertical zoning does something no single-floor flat can: it gives real separation between entertaining and rest without a single extra wall, using the staircase and the floor slab as the divider. Plan the flat around this split and the circulation almost designs itself.
Storage lives under the stairs
The staircase isn’t only circulation, it’s the best storage opportunity in the home. The volume beneath the stairs is prime built-in territory: made-to-measure joinery there turns dead structure into a full-height cupboard, a concealed store, a home for the things a two-floor household accumulates. Because it’s tucked into the spine of the plan, this storage costs no usable floor area anywhere else. Designing the stair and its understairs joinery as one piece, from the start, is one of the highest-return moves in a duplex.
The services quietly double
This is the budget reality people underestimate. In a duplex the mechanical and electrical services effectively double, because a second floor needs its own provision. Expect, in many cases, an extra plumbing riser to serve upstairs wet areas, air-conditioning zoned per floor so the two levels can be controlled independently, and weak-current wiring, data, TV, smart-home, run to both floors rather than one. None of this is exotic, but all of it is real cost and real coordination, and it belongs in the budget from day one rather than surfacing as a mid-job addition. A duplex priced as if it were a large single-floor flat is a duplex priced wrong.
Opening the space up is a structural question
The dramatic duplex moves, a double-height void, a sculptural feature stair, relocating the internal staircase, are exactly the ones that stop being interior design and become structural engineering. Adding or moving an internal staircase, or cutting a new opening in the floor slab, raises structural questions a contractor cannot answer alone. Any such change requires an Authorised Person (AP), a Registered Structural Engineer (RSE) and Buildings Department consent. The honest sequence is to raise these at the site measure and have the ambition checked for feasibility before you fall in love with a render, because the difference between “we can open this up” and “we can’t” is set by the structure, not the mood board.
Phase the works by floor
A two-level renovation is more coordination than a flat, and the way to keep it legible is to phase by floor. Sequencing the works so one storey progresses in a planned order relative to the other keeps trades from tripping over each other and keeps a large job readable week to week. It also lets the doubled services be run and tested floor by floor rather than all at once. This phasing is deliberate programme work, not something that sorts itself out on site, and it’s part of why a duplex takes longer than its area alone would suggest.
Budgeting for two floors honestly
Add it up and the conclusion is plain: a duplex costs more than a single-floor flat of comparable total area, and the premium is not padding. It comes from the staircase (structure plus understairs joinery), the doubled M&E services, and the extra coordination of working across levels. Budgeting a duplex well means naming those three costs up front rather than discovering them, and deciding early how ambitious the structural moves will be, because a feature void and a simple stacked layout sit at very different price points.
How Primeworks handles this
We design a duplex from the staircase outward, fixing the vertical circulation and its understairs storage first, then zoning public functions below and private above so the two floors read as one home. We plan the doubled services (an extra riser where needed, per-floor AC, weak-current to both levels) into the budget from the start, and flag any stair or slab opening as a structural question at the site measure, bringing in an AP and RSE before the design is committed. Carrying design and build under one accountable team, we phase the works floor by floor on a transparent, staged payment schedule tied to real progress.