100m² garden, no counted structures
The numerical range is 32–45m² because both area tests allow the statutory maximum.
How the calculation works
The provisional maximum is the lowest of 45m², the area left under the combined 45m² cap, and the area that leaves 25m² private open space. If that figure is below 32m², this calculator shows no numerical Class 3A range.
This is not an eligibility decision. Location behind the house, ownership and residence, access, height, setbacks, services, intended use, planning history, Building Control and every other condition still matter. Read the Class 3A legal-information guide and run the full checker.
Worked examples
100m² garden, existing 10m² structure
The combined-area test limits the potential range to 32–35m².
56m² open space
Leaving 25m² allows only 31m², below the 32m² minimum for this route.
Frequently asked questions
Does this confirm that I can build?
No. It checks only two numeric limits and cannot confirm planning status, exemption, certification or site suitability.
What if I already have a shed?
Enter the area that may count as a Class 3 structure. Its legal status and classification should be checked separately.
Reviewed 14 August 2026. Official context: local-authority Class 3A summary and notification guidance and National Building Control Office guidance. Always check current legislation and your site facts.