The starting position is that all development requires planning permission. In a domestic context, works that are classed as a development include things like the erection of walls, fences, porches and house extensions, making alterations to an existing house, the demolition of the whole or part of a building, altering a hedge or earth-bank, laying a drive or patio, and parking a caravan.
However there are instances where proposed works could still be classed as a development but also be exempt from the requirement to apply for permission. This is because not all development in Guernsey requires an actual application for planning permission. Changes to your home and garden that may not require planning permission are known as ‘exemptions’.
To come within an exemption, all of the conditions must be satisfied, which include strict requirements regarding dimensions, distances from the highway or a boundary with a neighbouring residential property, that the development isn't within a conservation area or involving a building built before 1900, and the proposed development must be within what's called your 'domestic curtilage'. ‘Domestic curtilage’ refers to the area of land on which your home sits and which is used for residential purposes, broadly what you would think of as your garden. It doesn't include a field or other open land next to a house, even if it is in the same ownership, because that land is usually considered agricultural land from a legal perspective.
Over 20 categories of domestic development are exempt from the requirement for planning permission including: the erection of a porch, shed, glasshouse, free-standing garage, garden structure, gates, fences, walls and some extensions to a dwelling-house. You can find an exhaustive list here.
Be aware that there may be conditions of previous planning permissions that affect exemptions, for example where planning permission for a group of houses was granted subject to a condition removing exemption rights for new walls or sheds. Bear in mind that there is a limit on the total area of exempt development which can be permitted within the curtilage of a house. No more than 50% of the curtilage, excluding the ground floor of the house as originally constructed, may be covered.
It's also worth noting that exemptions for dwelling houses don't apply to flats, maisonettes or self-catering accommodation and planning permission will usually be required in all cases for changes to Protected Buildings unless it specially says otherwise in the exemption. Don't forget planning exemptions don't apply to Building Regulations so you may still need buildings regulations permission for development that doesn't need planning permission.
If you aren't sure whether your proposed development comes within an exemption it is a good idea to seek professional legal advice and then arrange a pre-application meeting with the Development & Planning Authority planning officer.