One storey or two? What changes on the plot, in the heat and in the cost

"I want it on one floor, because stairs wear you out." "I want it on two, so the plot is big enough." Both are reasonable, and both decide far more than they say.

Going up or not changes how much of the plot the planning rules let you cover, how the structure is calculated, where the heat builds up, who can see whom and what the house can grow into in ten years. There is no answer that is right for every house. There is one that is right for your plot, your climate and the way you live, and it is easier to find with all five on the table before anyone draws.

The plot first: how much the rules let you cover

On urban land everything turns on two numbers that national law defines and each municipality fills in. The site coverage ratio (índice de ocupación) is the share of the plot that can sit under a roof at ground level: the shadow of the house on the ground. The floor area ratio (índice de construcción) is how many times the plot area can become built area, adding up every storey. Both are explained step by step in what can I build on my plot.

A single-storey house hits the coverage ratio first, because its whole brief has to fit in the footprint. With the illustrative numbers from that article (a 300 m² plot, coverage of 0.60 and floor area ratio of 1.50) it shows at once:

On a generous plot the coverage ratio barely matters and a single-storey house fits with room to spare. On a mid-sized urban plot it is the other way round: going up becomes the way to fit the brief and still leave ground for a courtyard, a garden or parking.

In Cali: the rules, and a staircase that counts

In Cali the ratios, the maximum height and the setbacks are set by the POT (the city's land-use plan, Acuerdo 0373 de 2014) by sector and planning treatment, so there is no citywide figure to give here. And that plan has an end date: it was annulled by the courts and remains in force under deferred effects until 9 July 2027, while the new one goes through (the deadline was 9 January, and the court extended it by six months because of the earthquake of 10 August). What that transition means for a permit is in what happens to your permit if Cali's POT changes.

One detail that only affects the two-storey house: Cali's plan does not generally exclude staircases from the floor area ratio, unlike Bogotá's. The stair area counts. In a house it is not hundreds of square metres, but on a plot already at its limit it can be the ones you are missing.

In the countryside: the slope matters more than the ratio

On rural land there is no floor area ratio but a housing density, and plots tend to be large: coverage is rarely what decides. The ground is. A long single-storey house on a hillside needs a long platform, with more cut and more fill; a two-storey one covers half the ground and can step its levels with the terrain. That is the studio's judgement, not a rule, and why fill is the enemy is in building on a sloping plot. What the rules allow in the countryside is in what can I build on rural land.

The structure: a second storey changes the calculation, not just the height

The NSR-10, Colombia's seismic building code, has a whole title for small houses: Title E, "Houses of one and two storeys". It covers confined masonry and cement-rendered bahareque (a framed earth-and-cane system), for residential use, in schemes of up to 15 homes and under 3,000 m². Your house falls inside it. Built with another system (concrete frames, steel, light-gauge steel framing), Title E no longer applies and the house is designed the long way, under Title A.

That one title covers both one and two storeys does not mean it asks for the same thing. For confined masonry, the standard Colombian house, the figures change like this:

The last row is the one that weighs most on the plan. On two storeys, the ground-floor walls carry the roof and the floor slab as well, and the minimum wall length is worked out from that sum. A smaller footprint, but more wall per square metre of ground floor.

The rule that shapes the design most

Translated into a floor plan: the bedrooms upstairs rest on the walls downstairs. And there lies the classic conflict of the two-storey house: downstairs you want an open living space, living, dining and kitchen in one sweep; upstairs you want rooms, which are walls. Making the two plans line up is design work from the first sketch, not something the engineer fixes at the end. The single-storey house does not have that problem: every wall runs from the ground to the roof.

Two things do not change between one storey and two. The soil survey: a house of up to three levels is a low-category building unit, with a minimum of three six-metre boreholes either way. And who signs: the structural design is signed by a civil engineer, whether the house has one storey or two.

The staircase, and growing old in the house

The staircase is the piece a single-storey house does without, and it is not small. It takes up area on both floors, organises how people move through both, and, as we saw, in Cali it adds to the floor area ratio.

The real question is another one: do you want to live in this house twenty years from now? A knee operation, a parent who moves in, or a wheelchair for a few months turn a staircase into a border. We found no rule requiring accessibility in a private single-family house, so what follows is the studio's judgement, not a requirement:

• —A possible bedroom on the ground floor, with a full bathroom next to it. Today it can be the study or the guest room; what matters is that someone can sleep and shower there without climbing.
• —A stair in straight flights, comfortable, with handrails on both sides. That is the kind that most easily takes a stairlift later, without being rebuilt.
• —A step-free entrance, or room for a ramp. In the countryside, on a slope, this is settled when the house is placed on the site, not afterwards.

The single-storey house solves all of that from the start. The two-storey one solves it if it is thought through from the beginning, and it usually costs little: it is a question of where things go.

The climate: heat rises, and the roof rules

At the Universidad del Valle weather station run by IDEAM, Colombia's national meteorological institute, in the south of Cali, the mean maximum temperature is 30.1 °C and the mean minimum 19.5 °C (1991-2020 normals). Hot afternoons and cool nights: the game for a house in this climate is to shelter by day and open up at night. One storey and two play it differently.

• —On one storey, every room is under the roof. With the high tropical sun, the roof is the surface that takes the most sun, and for the same floor area a single-storey house has twice the roof of a two-storey one. Everything depends on that roof insulating, venting or casting shade.
• —On two storeys, the ground floor is shielded by the floor slab, and the upper floor takes the whole roof. The heat that builds up rises, and the bedrooms, usually upstairs, are the hottest rooms by late afternoon.
• —The single-storey house ventilates sideways: cross-ventilation, eaves, a courtyard. The stack effect barely exists without height, as we showed with numbers in the central courtyard.
• —The two-storey house has an engine the other lacks: the staircase. With a high outlet it becomes a chimney: with twice the height and a little more temperature difference, the sum in that article gives four times the draught of a single-storey courtyard. It works mostly at night. See the staircase as a chimney.
• —Opening up at night is easier upstairs. An upstairs window can be left open to let the cool air in with less worry than one onto the garden. It is judgement, but it matters: night cooling only helps if the house can be opened.

Views and privacy

The upper floor sees over the boundary walls: the mountains, the valley, the trees. And whatever you can see, the neighbour can see from the other side. There is an old and little-known rule about that:

The distance is measured from the outermost line of the window or balcony to the boundary. On a narrow urban plot, that decides where the upper-floor windows can go, and it comes on top of the setbacks set by the land-use plan, which are a different rule and vary by sector. In a single-storey house, privacy comes from walls and courtyards. What is worth looking at is the reverse: if the neighbour has, or may build, two storeys, your courtyard is on view, and a well-placed pergola or tree is worth more than a tall wall.

The cost: what goes up and what goes down

We will not say that one is cheaper than the other per square metre, because there is no source to back it: no Colombian table with a published method exists, and why the ones in circulation are useless is in what it costs to build per m². What we can do is go item by item and see what moves. This is a qualitative comparison and the studio's judgement, for the same built area:

The honest reading is that they even out. A single storey saves the slab and the stair and pays for more roof, more foundation and more earth; two storeys save ground and roof and pay for a slab, a stair and height. On a flat, generous plot, a single storey tends to be the simple option. On an expensive, small or sloping one, saving ground is worth more. The full sum for a house in the countryside is in what a country house costs.

Growing later: planning the second storey in advance

It is the middle option and the one people ask for most: "one floor now, the second one later". It works, on one condition: that it is decided now. Adding a storey to a house that was not expecting it means analysing the whole building again because, as the NSR-10 says of vertical extensions, the two parts act together. The detail is in adding a second floor.

Planning for it means, in practice:

1. 1Foundations and walls designed for two storeys from the start: the two-storey figures in the structure table, not the one-storey ones.
2. 2The ground-floor walls placed where the upper floor will need them, because of vertical continuity. That is design, and it costs nothing if done in time.
3. 3The place for the stair reserved, with its opening in the slab allowed for, even if for now it is a storeroom below.
4. 4A roof that can become a floor slab, or a lightweight one that is easy to take down. A slab keeps the door open; the comparison is in the roofs article.
5. 5Services with their openings left: downpipes, drains and ducts for whatever goes upstairs.
6. 6The drawings and the structural calculations kept. They are the proof that the house was designed to grow, and the first thing whoever designs the extension will ask for.

Five questions before you decide

1. 1How much will the rules let me cover, and how much plot do I want free? If the brief does not fit in the footprint, the answer is already given.
2. 2Is the plot flat or sloping? On a hillside, less footprint means less earth to move and less retaining wall.
3. 3Who will live here in twenty years, and can they climb stairs? If the answer is unsure, a bedroom and bathroom downstairs, or a single storey.
4. 4Where do I want the cool air and where the view? Bedrooms upstairs with a stair chimney, or a house open onto a courtyard.
5. 5Do I plan to grow? If so, design for two storeys today, even if you build one.

These questions are answered with the plot in front of you, with the rules for that plot and before the first drawing, which is the moment of the schematic design. If you are planning your house in Cali, in Jamundí or in the Valle del Cauca countryside and want to put them to your own case, start with a free fifteen-minute first call.