The cheapest DIY urban planter box from pallets is a simple, no-bottom raised box made from one free heat-treated pallet. It costs almost nothing if you can find pallets, screws, and soil locally. The design is basic: cut four 12-inch corner posts from the pallet’s thick stringers. Cut pallet slats into side panels, each about 20–24 inches long and 10–12 inches high. Screw or nail two or three slats to each side of the posts to form a square or rectangle. Leave tiny gaps between slats for drainage and airflow. If you are on a balcony or roof, add a bottom made from extra slats, salvaged plywood, or a whole pallet as the base. Line it with burlap, old cotton fabric, or a food-safe liner, and drill a few drainage holes. Add small feet or bricks so water can escape.
For the cheapest fill, do not buy bags of potting soil. Use free compost, leaf mold, aged manure, topsoil from friends, coffee grounds, and crushed eggshells. Fill the bottom quarter with broken branches, twigs, and dry leaves to save soil. Then add a mix of compost and topsoil. This is basically a mini hugelkultur bed. Start plants from seed, cuttings, or swaps instead of buying nursery plants. Lettuce, herbs, chard, cherry tomatoes, peppers, and strawberries work well in a pallet planter.
Safety matters. Use pallets stamped HT (heat-treated), not MB (methyl bromide). Avoid pallets with oil stains, paint, or unknown chemicals. Sand rough edges. If you are growing food and the pallet history is unknown, line the inside with a food-safe barrier or use the pallet wood only as the outer frame. The total cost can be $0 if everything is scavenged, or about $5–$15 for screws and a liner. Soil can be free if you compost, or $20–$40 if you buy it. That makes this the cheapest pallet planter box for an urban garden: quick, upright, and made from waste wood.