The best way to store urban outdoor furniture in the off-season is to treat it like any other piece of outdoor gear: clean it, dry it completely, and give it protection that lets air move. In a city, space is usually the biggest issue, so start by deciding what can come inside and what has to stay out. Cushions, fabrics, and small accessories should go indoors—under a bed, in a storage bench, or in breathable fabric bags. Avoid sealed plastic, which traps moisture and encourages mildew.
Before storing anything, wash frames with mild soap and water, rinse, and dry thoroughly. Use a soft brush for crevices and wipe hinges and joints. For metal furniture, touch up chips and apply a little rust inhibitor; lubricate folding mechanisms. Wood furniture usually benefits from a clean and a light oiling if the manufacturer recommends it. Resin and plastic can simply be washed and dried. Fabric slings need mild soap, thorough rinsing, and full drying before storage.
If furniture stays outside, elevate legs off wet paving, group pieces together, and anchor them against wind. Use a breathable, fitted cover rather than a tarp, and leave a little ventilation so condensation can escape. Secure the cover so it doesn't flap, but don't wrap furniture tightly in plastic. Store folded chairs and tables vertically on wall hooks, behind a door, or in a slim balcony cabinet; stackable pieces should be separated with cloth to prevent rubbing. Keep cushions and straps indoors.
Check stored furniture once a month on a dry day. Wipe any condensation, look for mold or rust, and let everything air out. With a little prep and breathable protection, urban outdoor furniture can come through the off-season ready to use.