Case Study 1 - Plot Envelope, FAR and an Over-Confident Consultant
A developer owns a rectangular plot measuring 18 m by 24 m, on the corner of a 12 m wide road and a 9 m wide lane. The byelaws for this plot permit a maximum ground coverage of 40% and a floor area ratio of 1.8, with mandatory setbacks of 3 m at the front, 2 m at the rear and 1.5 m on each side. The height limit is 15 m and the client has budgeted for a floor-to-floor height of 3.3 m. The brief asks for six apartments of 110 sq m each, plus 15% of the apartment area for common circulation, a staircase and a lift core. The client's own consultant has already produced a scheme with five full floors built out to the setback line, arguing that the setbacks hand back the extra area. Before the drawing goes for sanction, the architect has to test the proposal against the byelaws and against the daylight and cross-ventilation the residents will actually get.
1. Calculate the maximum permissible built-up area on this plot and state how many full floors a compliant scheme can contain. Show every step and name the binding constraint.
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Step 1 - Plot area. 18 × 24 = 432 sq m.
Step 2 - Ground coverage. 40% of 432 = 172.8 sq m maximum footprint.
Step 3 - Buildable envelope after setbacks. Width 18 − (1.5 + 1.5) = 15 m; depth 24 − (3 + 2) = 19 m; envelope = 15 × 19 = 285 sq m. Because 172.8 < 285, coverage - not the envelope - is the binding limit on footprint.
Step 4 - Floor area ratio. 1.8 × 432 = 777.6 sq m total permissible built-up area. 777.6 ÷ 172.8 = 4.5 floors, so FAR alone would allow four full floors plus a half floor.
Step 5 - Height limit (the constraint the consultant forgot). 15 ÷ 3.3 = 4.54, so only 4 floors fit (13.2 m). A fifth floor would top out at 16.5 m and breach the 15 m limit.
Step 6 - Result. Maximum built-up area = 4 × 172.8 = 691.2 sq m, with the height limit binding before FAR does. The 777.6 sq m FAR entitlement is unusable on this plot; the architect should say so in writing rather than design to the FAR number.
Step 7 - Test the brief. 6 × 110 = 660 sq m; + 15% circulation = 99 sq m; total 759 sq m. This exceeds 691.2 sq m by 67.8 sq m, so the brief as written cannot be built legally. A workable adjustment: 6 units of 100 sq m = 600 sq m, + 15% = 690 sq m, which just fits. The architect must surface this now, not after sanction.
2. The consultant argues that the mandatory setbacks are 'free area' and has drawn five full floors out to the setback line. Quantify each byelaw breach and explain the error in the consultant's reasoning.
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Breach 1 - Setbacks. The consultant has built to the plot line, ignoring 3 m front, 2 m rear and 1.5 m side setbacks. The permitted envelope is 15 × 19 = 285 sq m, so the footprint of 18 × 24 = 432 sq m is 147 sq m over the envelope and 259.2 sq m over the permitted ground coverage - a footprint 2.5 times the legal maximum.
Breach 2 - FAR. Five floors × 172.8 = 864 sq m against a permissible 777.6 sq m, an excess of 86.4 sq m (about 11%).
Breach 3 - Height. Five floors at 3.3 m = 16.5 m against a 15 m limit.
The reasoning error. The consultant has treated the setback line as a permission and the buildable envelope as an entitlement. A setback is a subtraction from the plot, not an addition to it: the envelope is an outer boundary that the building may not cross, while ground coverage - a separate and here tighter control - fixes how much of that boundary may be filled. Two numbers govern, and the smaller one wins. The same confusion runs through the claim that setbacks 'hand back' area: the area between the plot line and the envelope is precisely the area the building may not use.
Practical consequence. Sanction will be refused and the drawings are wasted work. The architect should issue a one-page compliance note showing plot area, coverage, envelope, FAR and height in that order, so that every subsequent sketch starts from the correct cap.
3. Assuming a compliant footprint of 172.8 sq m, propose a plan proportion and placement that maximises daylight and cross-ventilation, with reasoning.
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The design problem. A 172.8 sq m footprint inside a 15 × 19 m envelope can be shaped in many ways, and the shape decides whether habitable rooms get useful daylight.
Option A - fill the envelope (15 × 11.5 m). Depth of 11.5 m means rooms 5.75 m from a window, roughly double the 4.5–6 m depth at which single-sided rooms still ventilate and day-light acceptably. Poor.
Option B - recommended slab (9.1 × 19 m). Place a slab 9.1 m deep along the 19 m side of the envelope, on one flank, leaving a 5.9 × 19 m open court inside the setback envelope. Every habitable room is then within about 4.5 m of a facade, and the 9.1 m depth allows genuine cross-ventilation from one long face to the other.
Why this works.
- The court is not wasted area: it is the daylight and air source for the deeper rooms, and it satisfies the rear and side setbacks rather than fighting them.
- Units can be planned front-to-back, so each gets a window on two opposite faces, which is what makes cross-ventilation possible.
- A 9.1 m structural depth suits a simple two-way or one-way slab grid and keeps the corridor single-loaded or short, so circulation stays near the 15% allowance.
Caveat the architect must state. The final choice depends on the sun path, which needs a site plan with north marked; on a west-facing long facade, the slab should carry balconies, fins or a service zone rather than unprotected glazing. Proportions should be argued from daylight and ventilation depth, not from facade appearance.