Use a project budget, not one price per square metre
Our calculator uses transparent editorial allowances for an insulated room, with separate amounts for foundations, services and contingency. These are planning assumptions rather than market averages or supplier quotes. Smaller rooms often cost more per square metre because doors, electrics and delivery do not shrink in proportion.
A real example of why scope matters
When checked on 9 September 2026, Woodbridge’s DIY price table listed a 3m × 3m package at £4,015 including VAT. The specification describes a panel-based shell package with openings for doors and windows. Confirm the final package directly: the page also contains different starting-price figures. That is not the budget for a connected, decorated office.
Installed ranges span a wide price band
Green Retreats’ comparison page displayed starting prices of £9,485 for its B-Range and £23,790 for its G-Range when checked on 9 September 2026. These are different products and specifications, not a like-for-like cost range. Check the current size, inclusions, promotions and site requirements before comparing.
Costs that are easy to miss
Allow for the job outside the room as well as inside it.
- Site clearance, skip hire, delivery access and ground preparation.
- Foundations designed for your ground and intended loads.
- Power connection, trenching, testing, heating and internet.
- Planning or building-control advice, drawings and fees where needed.
- Internal finishes, furniture, blinds and security.
- A contingency appropriate to the uncertainty; our example uses 15%.
Where to reduce cost thoughtfully
A simple footprint, modest glazing and a clear specification can help control cost. Compare local trade quotes and kit scopes before choosing. Avoid saving money by guessing structural sizes, removing moisture-control layers or doing electrical work without the required competence and certification.