The thing nobody tells you before you buy a backyard studio is that the foundation matters more than anything inside it. A slab or pier foundation that's properly leveled and sits above grade will keep the floor dry and the door swinging true for twenty years. A cheap one dropped on gravel or skids will shift within a season, and suddenly your door doesn't close right and there's a gap under the trim letting in every mouse in the neighborhood. Ask about the foundation before you ask about the windows.
A well built studio shows itself in the joints and the walls, not the paint job. Look for actual timber framing, two by four or better, spaced properly, not just a plywood box with siding stapled on. Check how the roof meets the walls, whether there's a real overhang to keep water off the siding, and whether the windows are double glazed or just single pane glass that will sweat and fog the first cold morning you're in there with a coffee. Insulation is the other thing that separates a studio you'll use in January from one that's a glorified shed you abandon by October, so ask what's in the walls and the roof, not just what's on the spec sheet. The honest limits are worth saying out loud too: if you want running water, a toilet, or serious heating and cooling, you're now talking permits, a real electrician, maybe a plumber, and a budget that's crept well past what the brochure implied. A backyard studio is brilliant for a home office, a studio for painting or pottery, a reading room, a place for a teenager to have somewhere that isn't their bedroom. It's a poor substitute for an extension if what you actually need is more bedrooms or a kitchen, and it's a miserable choice if your yard faces north and gets no light past eleven in the morning, because no amount of good design fixes a room that's dim all day.
Getting the size right means walking the actual footprint with string or tape before you order anything, not just eyeballing the yard. Measure the clearance to your fence and your neighbor's windows, because most areas have setback rules that will bite you if you guess. Think about where the sun actually falls in the afternoon in summer versus how low and weak it is in December, because that's the difference between a studio that's glowing and usable in the evenings and one that's a hotbox from May to September. People consistently buy these too big for the plot and too small for what they actually wanted to do in them, so stand in the space with a chair and picture the exact working day before you commit.
Would I actually walk out here on a wet Tuesday in February, or is this only a yes in July?