How high can you go in a single day from sea level before altitude becomes a real concern?

General guidance rather than anything specific to you — individual response varies a lot and anyone with a relevant medical history should ask a doctor rather than a forum.

The commonly cited threshold where altitude illness starts to appear at all is around 2500 metres, and your 2700 is just above it. At that height, in an ordinary healthy person arriving for a short time, symptoms are usually mild if they occur — headache, poor sleep, being out of breath more easily than expected, reduced appetite.

The standard acclimatisation rules — limiting the gain in sleeping altitude per day, building in rest days — are aimed at going considerably higher and staying there. For a day trip to 2700 they are not the relevant framework.

What matters far more at your altitude is where you sleep. Going up for the day and coming down to sleep low is a much smaller ask than sleeping at 2700 on your first night.

The practical things that make a day like this go well, none of which require any special preparation:

• Go up gradually within the day if you can. Driving straight to the top and immediately walking uphill is the harshest version.
• Drink more than you think. Dehydration at altitude is easy and its symptoms overlap almost perfectly with mild altitude illness, so people misattribute it constantly.
• Walk slowly. Deliberately slower than your sea-level pace. Most of the misery people report at moderate altitude is from going at their usual speed.
• Avoid alcohol the night before and during.
• Eat, even without appetite.

The rule everyone should know: if symptoms are getting worse rather than better, go down. Descent is the treatment for altitude illness and it works quickly. Nothing about a summit is worth ignoring that.

One thing worth knowing because it surprises people: susceptibility is not well predicted by fitness.

Very fit people get altitude illness, and unfit people frequently do not. The best available predictor is your own history — how you have responded at altitude before. Since you have not been up, you have no data, which is an argument for a conservative first outing rather than for worry.

There is also a common pattern where fit people fare worse in practice, for a behavioural reason rather than a physiological one: they go faster, higher and further because they can, which is exactly what you should not do on the first day at altitude.

Worth planning the trip so descending is easy. On a drive-up mountain that is straightforward — the car is at the top — but check the return timing.

The common failure is being three hours' walk from the road at the point where somebody starts feeling bad, at which stage descending is a decision with a real cost. Keeping the first day's route short and reversible removes that entirely and costs nothing but a slightly less ambitious day.