Adding a second floor: what it costs, and whether the house can take it

The conversation always opens the same way: «I want to add a second floor, what does the square metre cost?». And that question, though it looks like the first one, is the second.

The first is whether the house can take it, and it is not rhetorical: the seismic code devotes an entire chapter to intervening in what is already built, and that chapter decides whether the works cost twenty or sixty. Any square-metre price quoted before that answer is pricing a job nobody has defined.

The chapter governing this is called A.10

NSR-10 does not treat an existing building as new work, and it leaves nothing to anyone’s judgement. Its Title A carries a specific chapter, and its opening paragraph states its purpose:

And in case there were any doubt that your case is inside it, A.10.1.3.4 names it outright: the criteria of the chapter «shall be used in the design and construction of extensions alongside or extensions upwards». It is not a grey area. It is a written procedure.

Growing sideways and growing upwards are not the same job

This is the distinction almost nobody explains to an owner, and it is the one that moves the most money. The code splits extensions in two and gives them different requirements.

Alongside: there is a way out

An extension alongside is one «in which the area is enlarged without altering the height». And there the code opens a door that the upward extension simply does not have:

Upwards: there is no way out

An upward extension is one «in which the height of the building is altered, with or without an increase in floor area». Here a joint is no help, because the new load has to travel down through the old structure:

Four duties follow from that, and owners never budget for them:

• —Analyse the complete building afresh, old and new together, and show it resists what the code demands today, under vertical loads and horizontal forces alike.
• —Strengthen the old part if it falls short. If the combined indices come out above unity, «additional elements must be provided». It is not optional and it does not turn on the designer’s taste.
• —Prove the connections. Junctions between new and old members have to show, by analysis or by testing, that they transfer the forces.
• —Bring the foundation into the sum. A.10.8.6 requires it to be investigated under the supervision of a geotechnical engineer, for bearing capacity, for settlement and — this is the forgotten one — for overturning.

The 10% rule, which almost nobody knows

There is a third category, modifications that are not extensions, and it carries the chapter’s only numeric threshold. Worth knowing before knocking a wall down:

In plain terms: a small internal alteration does not force a review of the whole house. Going past that ten per cent does. And whoever decides it was passed is not the site foreman.

What has to be found out about the house already there

A.10.2 lists the prior investigations, and they are worth reading because they explain why the assessment costs what it costs: nearly everything has to be discovered rather than looked up.

• —Where original structural and foundation drawings exist, check on site that they match what was built, with exploration at representative points and a record of its extent.
• —Rate the quality of the original design and construction, judged against the best technology of the period when it went up.
• —Rate the state of preservation of the structure.
• —Look for evidence of local failure: excessive deflections, corroded reinforcement and other signs of behaviour.
• —Investigate whether the foundation has settled and what effect that had.
• —Investigate extraordinary past events — explosion, fire, earthquake, earlier remodelling, finishes that added load — that may have affected its integrity.

And the outcome of all of it is two numbers, the ones that really answer the question in the title. The inverse of the general overstress index «expresses the vulnerability of the building as a fraction of the strength a new building would have» under today’s code. The flexibility index does the same with stiffness. Below one, the house takes it; above one, it does not.

The 2023 decree that gets misquoted

In August 2023 the government partially amended NSR-10 through Decreto 1401 de 2023, and in September corrected a slip in it with Decreto 1580 de 2023. What came in was a new document, AIS 410-23, on assessing and reducing seismic vulnerability in masonry dwellings. It matters here because almost every Colombian house wanting a second floor is exactly that: masonry.

The document is meant «for use in dwellings of informal origin […] and no more than three storeys in total», and it carries a table of limits that works as a reality check:

And NSR-10 is still NSR-10

Worth saying, because the opposite gets repeated. The code in force is the one adopted by Decreto 926 de 2010 and amended by Decretos 2525 de 2010, 092 de 2011, 340 de 2012, 945 de 2017, 2113 de 2019, 1711 de 2021 and 1401 de 2023, with the correction in 1580 de 2023. The trade update circulating as «the new code» still has no decree adopting it, and until it does no permit office can be asked to apply it.

The permit: which one, and the requirement that stops everything

Extending calls for a building permit, in a modality with a name of its own. The definition matters because it carries a limit many people discover late:

In other words: NSR-10 says whether the house takes it and the local plan says whether the plot allows it. They are two separate permissions dressed as one, and they can come out the wrong way round — a house structurally able to carry the floor, on a plot that has already used up its floor-area ratio. What the local plan looks at is in what you can build on your plot.

• —Validity: 24 months, extendable once by a further 12. The 36 months quoted everywhere belong to new build; extensions are not in that group.
• —Structural drawings, signed and titled. In low and medium complexity projects a copy of the structural drawings suffices; in medium-high and high complexity, calculation reports, non-structural element design and a geotechnical study must be filed.
• —In a condominium, minutes of the competent body authorising the works, and a notice 30 × 50 cm on the building noticeboard. How that gets decided inside is in altering a flat in a condominium.

What the procedure costs, with the sum worked out

This part does have a number, because the decree publishes the equation and the factors. Permit-office fees are charged as a fixed charge plus a variable charge, both in UVT, adjusted by income bracket, by municipality and by area. For an extension of up to 100 m² the area factor is a published value — 0.45 — and the sum closes on its own.

What the works cost: why there is no figure here either

I am not going to give a square-metre value for an extension, and the reason is the usual one: no published Colombian source carries a methodology, and here the problem is worse than in new build, because the heaviest line item — how much strengthening is needed below — is unknown until somebody assesses it. Why the tables in circulation are useless is set out in what a square metre costs.

What can be said about the shape of the cost:

• —The new floor is the predictable part. It is new work over a known area, and the only piece a builder can price seriously from the outset.
• —Strengthening the existing fabric is the part that varies tenfold, and it cannot be priced before the assessment. Pricing it earlier is pricing a problem nobody has defined.
• —The foundation can be the bigger surprise. Underpinning is work beneath the house, with the house on top of it: expensive by definition, and dependent on the soil.
• —Add living through the works, or not living in them. An upward extension takes the existing roof off; temporary relocation is a real line item and never appears in a quotation.
• —And the pre-construction stage is not optional. Survey, vulnerability assessment, geotechnics where the diagnosis calls for it, design of the strengthening and the permit. All of it happens before the first bag of cement.

If what you want is to bound the order of magnitude before signing, the cheap route is the usual one: assessment first, and with the diagnosis in hand, quotations that at last compare the same thing. It is the same mechanism that blows budgets when the step is skipped, told in why building work overruns its budget.

The order that works

1. 1Check the plot’s development potential first. If the local plan says no, the structural conversation is moot. It is a planning enquiry, and it is quick.
2. 2Gather whatever exists on the house: original permit, architectural and structural drawings, title deed. Where there is no permit, recognition comes before anything else.
3. 3Survey what is built, with real measurements rather than the ones on the deed.
4. 4Vulnerability assessment, signed by a structural engineer. This is where the two indices appear and where the amount of strengthening becomes known.
5. 5Geotechnical study, which in an upward extension is the rule rather than the exception, because the foundation necessarily enters the analysis.
6. 6Integrated design, architectural and structural at once. A second floor drawn without the structure over it gets redesigned from scratch.
7. 7Permit in the extension modality, and works supervised by whoever designed them.

So the question in the title has an honest answer in two halves. The procedure costs what the table above says, and can be known today. The works cost whatever the assessment says, and anyone giving you that figure before assessing is not pricing your house: they are pricing an average.