Every October the same thing happens: a clear evening, a forecast that says 4 °C, and a balcony full of pots you are not sure about. The honest answer is that "too cold" depends on the plant. Some shrug off a frost, some sulk at 10 °C. The trick is to know which kind each of your plants is, then glance at the forecast.
Slide the temperature. Tap a pot to carry it indoors (or back out) and watch what changes. The labels are rules of thumb, not guarantees.
Tender plants are houseplants that spend summer outside, like a monstera. They are not frost plants at all: one source puts the lowest temperature a monstera tolerates without significant damage at around 10 °C, and cold soil makes roots less able to work. If nights are heading towards single digits, they come in.
Frost-tender plants such as geraniums (pelargoniums) cope with cool weather but not with frost. The Royal Horticultural Society advises bringing container plants under cover before the first autumn frosts. A light, airy, frost-free place is what they want, and a cool room around 7 to 10 °C suits plants you want to keep flowering.
Tough plants like rosemary or strawberries can often stay out, but a pot is a harsher home than a garden bed. The roots sit in the air instead of the ground, so they can freeze from every side. A pot pushed against the house wall, wrapped in something, or lifted off cold concrete is usually enough for a light frost. Before a deep freeze, move the pot to shelter.
Use the lowest temperature, not the daytime one. A sunny 12 °C afternoon can become a 3 °C night, and a balcony in a clear sky loses heat quickly. Check the forecast a couple of days ahead, since carrying a large pot is easier when you are not doing it in the dark.
Bring plants somewhere bright and not too warm: a window sill in a cool room, a stairwell or a frost-free porch. Check them for pests before they come in, and water less, because they grow slowly in winter. Rooms that are bright but cool beat dark, heated ones for most of these plants.
No room? You do not need to bring everything. Pick the plants you would miss most, bring those, and let the cheap summer annuals finish their season. Cosy is not a rule about keeping every plant alive.
Measure the doorway and the path from balcony to spot first, including turns and stairs, so a big pot does not get stuck halfway. If a pot is heavy, lift with a friend or slide it on a cloth. Keep pots off a radiator and away from draughts when they come inside.