It's taken me a while to get my head around this.
Mainly, this is the upstairs flat in a house that is split into two flats, but the house has a garage to the side and the garage belongs to this the top flat.
The element of the property on the ground floor is just the front door and passageway to the stairs up to the main flat, plus the garage. The owner is offering that if desired the garage will be converted into an ensuite bedroom if the purchaser so wishes (does one need planning permission for that? Garage conversions like this are often really badly insulated) so the floorplan shows both the "garage" and "bedroom" options for this bit.
The main flat is on the first floor. It consists of a large bedroom with 2 en-suites and two dressing-areas, plus a lounge, kitchen and tiny dining room (which could optionally be used as a further bedroom if you didn't need a dining room). The large bedroom could optionally be split into two smaller bedrooms each with their own bathroom, so this is illustrated in the floorplan again as a potential different layout.
Above the lounge and kitchen is a glass-pannelled mezzanine walkway giving access to two little loft-spaces under the eaves at the front and back, which are called respectively a "snug" and a "library" - these are small single-bedroom size rooms but don't meet building-regs standards for being called bedrooms because their "fire escape" route is too complicated and can't be fire-protected, so they can't be described or sold as bedrooms.
For comparisson, very nearby there is this 4 bedroom house on the market for only £600,000 and the decor is much less awful. www.rightmove.co.uk/properties/128472803