Most garden rooms can comfortably be built under a householder's permitted development rights, though this can depend upon where you live, if your house is listed, and how tall you want your garden room to be, amongst other factors.
As mentioned elsewhere, it's always worth checking with your local planning department to make sure you understand if the garden room you want to build meets the relevant criteria.
The following links will take you to the official goverment guidance on permitted development across the regions of the UK. Generally they are all very similar but there may be some minor differences so do make sure you understand what's what. Our article about whether or not planning permission is required is also worth a read if you haven't already done so.
England
Permitted development rights for householders: technical guidance (HTML)
Link takes you to the section called Class E which relates to outbuildings.
Permitted development rights for householders: technical guidance (PDF)
The section related to outbuildings, Class E, begins on page 41.
Scotland
Summary guide to permitted development (14 Aug 2023)
Guidance on Householder Permitted Development rights publication (1 Apr 2021)
Outbuildings start in section 4.74.
Wales
Planning: a guide for householders (May 2020)
Outbuildings start at Section C, page 14
Permitted development for householders v2 - technical guidance (April 2014)
Outbuildings start in section 11, page 52.
Northern Ireland
The Planning (General Permitted Development) Order (Northern Ireland) 2015 (Apr 2015)
Outbuildings come under Class D which starts on page 15
As mentioned elsewhere, it's always worth checking with your local planning department to make sure you understand if the garden room you want to build meets the relevant criteria.
The following links will take you to the official goverment guidance on permitted development across the regions of the UK. Generally they are all very similar but there may be some minor differences so do make sure you understand what's what. Our article about whether or not planning permission is required is also worth a read if you haven't already done so.
England
Permitted development rights for householders: technical guidance (HTML)
Link takes you to the section called Class E which relates to outbuildings.
Permitted development rights for householders: technical guidance (PDF)
The section related to outbuildings, Class E, begins on page 41.
Scotland
Summary guide to permitted development (14 Aug 2023)
Guidance on Householder Permitted Development rights publication (1 Apr 2021)
Outbuildings start in section 4.74.
Wales
Planning: a guide for householders (May 2020)
Outbuildings start at Section C, page 14
Permitted development for householders v2 - technical guidance (April 2014)
Outbuildings start in section 11, page 52.
Northern Ireland
The Planning (General Permitted Development) Order (Northern Ireland) 2015 (Apr 2015)
Outbuildings come under Class D which starts on page 15