What Does It Cost to Build a Garden Room?

A realistic breakdown of foundation, shell, insulation and finishing for a small detached outbuilding.

A 25 to 30 square metre garden room lands in a wide cost range depending almost entirely on how it is insulated and finished, not on its footprint. The shell is a surprisingly small share of the total. Budget roughly in quarters: foundation and groundwork, structural shell and roof, insulation and weathertightness, then internal finishing, services and fitting out. The last quarter is the one that overruns, because it is where individual choices about flooring, glazing and heating accumulate.

Where the Money Actually Goes

Groundwork is the least visible and least skippable cost. Pad or strip foundations below the frost line, a damp-proof membrane and insulation under the slab are all invisible when finished and all extremely expensive to add later. Glazing is the other big lever: large sliding doors can cost more than the entire timber frame. Deciding early how much glass you want changes the budget more than almost any other single choice.

If the building is intended for year-round use rather than summer storage, insulation and airtightness move from optional to central. An uninsulated garden room is a shed with better cladding, and heating it through winter costs more each year than the insulation would have cost once.

Check the Rules Before You Cost Anything

Permitted size, height and boundary distance for outbuildings vary by country and often by municipality, and the thresholds usually change the moment the building has plumbing or is used for sleeping. Confirm the rules that apply to your plot before finalising dimensions. Redesigning on paper is free; discovering the limit after the slab is poured is not.

Expert Tip

Run a service duct out to the building during groundwork even if you have no plans for water or a dedicated electrical circuit. An empty conduit under a finished slab costs very little now and saves excavating a trench across a finished garden later.

Summary

Costs concentrate in groundwork, glazing and insulation rather than in the frame. Decide the intended use first, because year-round use changes the specification throughout. Use our Concrete Calculator for the foundation volume and the Insulation Calculator for the walls and roof.