A garden room extension attached to house is an extension, and the three decisions that shape it all involve the existing wall. How large an opening you make, what carries the wall above it, and where the insulated envelope of the house now ends. Get those three settled on a drawing before anybody quotes, and the quotes become comparable.
Garden room extensions: the size of the opening drives everything
A pair of doors into an existing opening leaves the house structure alone. A full width opening turns the rear wall into a beam supported at each end, which needs a structural engineer's calculation, padstones sized for the load, temporary propping while it goes in and building control inspection at the right moments. Between those two extremes the cost is close to continuous.
Where the thermal envelope now runs
Attaching a room to the house moves the line between inside and outside. The old external wall stops being an external wall, its insulation and its damp proof course have to be dealt with at the junction, and the new roof and walls become the thermal elements. Cavity trays where the new roof meets the old wall are the detail that keeps water out for the next thirty years.
The neighbours and the boundary
Building close to or up to a boundary brings the Party Wall etc. Act into play, both for excavating near a neighbour's foundations and for work to a shared wall. Notice periods are real and they sit on the critical path, so the notices go out early rather than when the digger is booked.
Heating a garden room house extension, because a cold one spoils a warm house
A room opened into the house has to be heated to the same standard or it becomes the cold end of the living space and the doors get shut again. That means extending the wet system, laying underfloor heating in the new slab, or accepting a separate unit, and the first two are decided before the floor goes down.
Garden rooms attached to house stop being outbuildings
This is the distinction that catches people. A garden room standing free in the garden is an outbuilding, planned under the outbuilding permitted development right and frequently exempt from the building regulations. Attach it to the house and it is an extension: planned under the extension rules with their own limits on projection and height, and assessed by building control as an extension, with the thermal, structural and fire requirements that brings. The building can look identical either side of that line and the process is not the same, so the attachment is a decision rather than a detail.
The lean to garden room extension, and why it is the common one
Where the garden room runs along the back of the house with a single roof plane falling away from it, the structure is simpler and cheaper than a freestanding building with its own four walls, because one wall already exists and carries the roof. The trade is that the existing wall now has a roof abutting it, which means a flashing and possibly a box gutter, and that junction is the thing to get right. Where the new roof lands below existing windows, their position drives the roof height and sometimes the whole design.
Questions people ask about garden room extension attached to house
Do I need an architect?
You need a drawing and a structural calculation, and who produces them varies. A design and build contractor can arrange both, and on anything with a large opening or an awkward junction an independent designer is worth the fee because the drawing is what the quotes are priced against.
How long does it take?
Longer than a detached garden room because the trades are sequential and the house is open at one point. Groundworks, structure, roof, glazing, and then the internal work each wait on the one before, and the opening is usually formed late for good reason.
Will it need a party wall notice?
If you excavate within the distances the Act sets out from a neighbour's structure, or work on a shared wall, then yes, and it applies even where planning permission is not needed. It is a separate process with its own timescales.
Can a garden room be attached to the house?
Yes, and it then counts as an extension rather than an outbuilding, which changes both the planning route and the building control position even though the building may look the same.
Is attaching it cheaper than a freestanding building?
The structure can be, because one wall already exists and carries the roof. The process is more involved, because it becomes an extension with the approvals and thermal standards that brings.