Whether you need planning permission depends on the type of annexe, its size, and how independently it functions from the main dwelling. A simple garden room used as a home office is likely to fall under Permitted Development, provided it meets size and position limits. A fully self‑contained annexe with its own kitchen and bathroom will typically require a full planning application because it effectively creates a separate dwelling unit. Ancillary accommodation — where the space is used by a family member and remains dependent on the main house — sits in between, and planning authorities will assess each case on its merits. We always recommend discussing your plans with your local planning authority before work begins, and we can manage the entire planning process for you.

A garden room is typically a single-use detached structure such as a home office, gym, or studio. It doesn’t usually include sleeping accommodation, a kitchen, or a bathroom as its primary function. An annexe is more substantial, designed as living accommodation with bedrooms, bathrooms, and often a kitchen. The key distinction for planning purposes is whether the space functions as an independent dwelling or remains ancillary to the main house. This distinction determines whether you need planning permission, which building regulations apply, and whether council tax is affected.

It depends on how the annexe is used and how it is assessed by your local authority. If the annexe is occupied by a family member and is not independently let, some councils may offer a discount or exemption — often referred to as the “Class W” or “granny annexe” exemption. However, this is not automatic and varies by council. If the annexe is let commercially or assessed as a separate, independent dwelling, it may receive its own council tax band. We recommend checking with your local authority early in the planning process so you understand the implications before committing to a design.

All annexes that include habitable rooms must comply with Building Regulations. This covers structural integrity, fire safety (Part B), thermal efficiency (Part L), accessibility (Part M), sound insulation, electrical safety (Part P), ventilation, drainage, and water efficiency. Our modular systems are designed to meet or exceed current Part L thermal standards as part of the build process, and we manage all inspections and provide final certification so you don’t need to coordinate with building control yourself.

A modular annexe typically takes 12 to 20 weeks from initial enquiry to completion. This includes design, planning (if required), factory manufacturing, and on-site installation. The on‑site work itself can be as little as 2 to 4 weeks, with the building watertight in 2 to 3 days once assembly begins. By comparison, traditional brick‑built annexes generally take 6 to 9 months and involve significantly more disruption to your home and garden.

Potentially, but space and planning constraints will be important factors. Many local authorities require a proportion of the original garden to remain unbuilt — often around 50%, though this varies by council. A well‑designed compact annexe of 20–30m² can fit in many gardens if planned carefully, but it is important to check your council’s specific requirements early. Site access for delivery and construction also needs to be considered, particularly for modular builds.

Potentially, but doing so may have implications for your planning permission and council tax. Many annexe planning approvals include a condition limiting occupancy to a family member or preventing the annexe from being sold or let independently. If you intend to let the annexe commercially, you would typically need full planning permission for a separate dwelling, and the annexe would likely receive its own council tax band. If renting is part of your long-term plan, it is worth making this clear at the planning application stage so the right permissions are in place from the start.