A garden room extension and a garden room are two different buildings and the word is used for both. The moment a garden room touches the house and opens into it, it stops being an outbuilding and becomes an extension: the building regulations apply in full, the wall it attaches to becomes structural work, and the permitted development rules that apply are the ones for extensions rather than the ones for outbuildings.
What changes the day it touches the house
A detached outbuilding can be exempt from building regulations approval on size and use grounds. An extension never is. Attached means thermal elements to the current standard, structural design for the opening, ventilation, and a completion certificate at the end. It also means the extension permitted development limits on depth, height and eaves apply, not the outbuilding ones.
The opening is usually the biggest single cost
Connecting the two spaces means removing part of the rear wall, which is a beam, a structural calculation, temporary propping and building control inspection. That single item regularly costs more than the difference in the shell, and it is the thing people forget when comparing an attached room with a detached one.
What you get for it
Habitable floor area that counts. An attached, insulated, signed off room is part of the house for a valuer, a lender and a buyer in a way a detached garden room is not, and it is warm because it is inside the house's thermal envelope rather than a separate one.
When detached is genuinely better
If the point is separation, an office you can leave at the end of the day, a gym you do not want to hear, a space that is not the house, then detached is the better building and the cheaper one. It also leaves the house's rear elevation alone, which on some houses is worth a great deal.
Questions people ask about garden room extension
Is an attached garden room the same as a conservatory?
Only if it is mostly glazed and separated by external quality doors, which is what the conservatory exemption requires. A garden room extension with solid walls, an insulated roof and an open connection to the house is an extension and is treated as one.
Can I attach a garden room to the house later?
Sometimes, and the base and the structure have to have been designed for it. More often the honest answer is that a building designed as a detached outbuilding will not meet the standards an extension has to meet without substantial work.
Which needs planning permission?
Both may be permitted development and the tests are different. Extensions have their own limits on depth from the original rear wall, height and eaves, and outbuildings have theirs on height, boundary distance and coverage. Check the right set for what you are building.