An L shaped garden room costs more per square metre than a rectangle of the same area, and the reason is corners. Every corner is a structural junction, a cladding detail, a break in the insulation line and a place for water to be directed. An L also creates an internal corner in the roof, which is a valley, and valleys are where roofs leak. None of that makes an L wrong, but it should be chosen for a reason.
Perimeter, not area, drives the price
An L shape has more external wall than a rectangle enclosing the same floor. More wall is more framing, more insulation, more cladding, more corner trims and more airtightness detailing. On a small building where the wall is a large share of the cost, that difference is significant and it is invisible if you compare on floor area.
The valley is the detail to ask about
An L plan roof has an internal corner where two slopes meet and discharge into each other. That valley carries a lot of water into one place, and it has to be formed properly with the right membrane and the right outlet. Ask how it is being detailed and where the water goes, because this is the single most common failure point on a shaped roof.
Two good reasons to build an L
The first is a site constraint: a tree, a manhole, a boundary or a change in level that a rectangle cannot avoid and an L can wrap around. The second is separation of use, where the two legs give a working end and a quiet end without a partition doing all the work. Both are real reasons; wanting an interesting shape is not.
What an L does to the inside
The internal corner is dead space unless it is planned for. It suits storage, a kitchenette or a WC pod, which is often exactly what people want, and it is poor for a desk or a bed. Draw the furniture before settling the plan, because an L that has not been drawn out ends up with an awkward corner nobody uses.
Questions people ask about l shaped garden room
How much more does an L shape cost?
There is no single figure because it depends on how pronounced the L is, but the direction is reliable: more perimeter, more corners, a valley in the roof and a more complicated base all push the same way. Ask for both shapes to be priced if you are unsure.
Is the roof more likely to leak?
A valley is a higher risk detail than a plain slope, and a well built one is not a problem. The question to ask a supplier is what membrane and what outlet detail they use at the valley, and how they have dealt with it on other buildings.
Does the shape affect planning?
Not directly. The outbuilding tests are about height, position and how much of the garden is covered, and an L shaped footprint counts its own area like any other. What it can affect is how close a leg gets to a boundary, which does change the height limit.