Garden Room and Annex Rules in Merton

The post-pandemic explosion in home working has seen a massive surge in outbuildings across the large, leafy gardens of Wimbledon, Raynes Park, and Morden. While detached gyms, studios, and home offices are highly possible under Permitted Development (PD), crossing the subtle line into a "self-contained annex" immediately activates rigorous planning enforcement.

Merton Council actively polices the primary use of outbuildings to prevent the stealth creation of separate dwellings.

The Boundary Between Studio and Dwelling

An outbuilding remains PD only if its use is "incidental" to the enjoyment of the main house:

• The Volume and Placement Limits: Under PD, the outbuilding must be single-story, with a maximum eaves height of 2.5 meters. If it sits within 2 meters of the boundary, the entire building cannot exceed 2.5 meters. Furthermore, it must never sit forward of the 'principal elevation' (the front of the house); putting an office in the front garden requires Full Planning.
• The Primary Use Test: A home gym, a snooker room, or an occasional home office is "incidental." Primary living spaces—such as a kitchen, a primary bedroom, or a fully plumbed bathroom (rather than just a simple WC)—are not incidental. Including shower trays and cooking facilities is the quickest way to have the building reclassified as an illegal separate dwelling.
• The Annex Exception: If you seek to build a "Granny Annex" for a family member to sleep in, it is no longer PD. You must submit a Full Planning Application. Planners will only approve it if a clear, functional dependency firmly links the annex to the main house (e.g., shared utility meters, shared garden access, and no separate postal address).

Official Merton Council Resources

Before committing to any major architectural project, we strongly advise cross-referencing your ambition directly with the local authority. The following links provide direct access to Merton Council's live planning portals and heritage registries:

• Merton Planning & Building Control Portal
• Search Live Merton Planning Applications
• Merton Heritage, Conservation Areas & Article 4 Directions

How We Can Help

If you are considering a major refurbishment, extension or basement in Merton, our in-house architectural and construction teams are highly experienced with the specific constraints and policies of this council. Do not leave your planning application to chance—our Planning & Permissions and Architecture services are explicitly designed to handle strict London authorities from initial conceptual design through to final, legal consent.

Once permission is secured, our Refurbishment & Interiors division carefully manages the execution, guaranteeing the design integrity is maintained throughout the build phase.

*Published in the Hampstead Renovations Planning Guide Collection — delivering expert design and build strategies for London's most heavily guarded conservation boroughs.*