An important consideration when evaluating the cost of a garden room is the effect it has on the overall function of your garden. With a standard garden room design, your garden room will consume a third of your garden. It is extremely difficult to accurately assess the effect of a design element on your garden from plans and elevations.
In the UK, the average size of a garden is approximately 7 meters (23 feet) by 5 meters (16 feet). A standard garden room design of 3.5 meters by 3.5 meters consumes 10.5 square meters. With a design that consumes 10.5 square meters of your garden, it limits the use of the remainder of the garden and creates an obstruction for the rest of the garden which remains unsheltered and untended.
A design that takes the same square meterage consumes less of the depth of a garden and can make a bigger positive effect on the overall garden design. For example, if you took a design that gives the same square meterage as a garden room that is located 3.5 meters from a side of the house and placed it on the other side of the house, it makes a bigger positive effect on the overall garden design and maintains the sight line of the garden.
Building a garden room has a lot of trade-offs. Depending on the rules in your area, you may not have much of a choice in design. For example, a roof no more than 2 meters from a property boundary must be no higher than 2.5 meters. This means the eaves of the roof must be even lower, making a roof with a higher peak (like one that is common on a garden shed) non-compliant.
It is also important to understand the costs and benefits of designing a garden room. For example, a garden room that is 2.5 meters wide and 3 meters long (7.5 square meters) takes up less of your yard, but may obstruct a workspace and storage. On the other hand, a garden room that is 3.5 meters wide and 3 meters long (10.5 square meters) gives you enough space to have a workspace and storage, but may reduce your yard space even more. Designing a garden room may be more costly if you have a smaller yard.
Glazing alters a design in ways that are often difficult to recognise. A garden room with unobstructured glass wall facing the garden captures a part of the adjacent space and lessens the sense of boundary between the two. In comparison, a standard room with a smaller total glazed area, in addition to a greater area of cladding, creates a stronger sense of boundary. Although the total space enclosed by the two designs is the same, the design with more glazing creates a smaller sense of boundary and gives less recognition to the loss of space.
There is no absolute definition for the maximum amount of a small garden that can be built upon. It depends on the function of the garden. A garden that is fully enclosed by the built space and essentially becomes part of the room allows for a greater built space compared to a garden that is partially built upon and reduces the function of the space (e.g. play space for children). In effect, the greater the function that is lost from the built space, the more careful one should be in evaluating if the built space is integrated with the main function of the room.