There is a real product behind this search and it is not a portable. A monoblock wall unit puts the whole refrigeration circuit inside one case on an internal wall and vents through two cored holes to the outside, so there is no outdoor box, no refrigerant pipe run and no F gas handling at installation. That combination solves a specific set of problems and creates a different set.
What it is and how it differs from a split
A split system separates the noisy, hot half of the circuit and puts it outside. A monoblock keeps both halves in one case on the wall inside, and exchanges heat with the outside air through two large cored ducts. The refrigerant circuit is sealed and factory charged, which is why installation does not require the same certified handling that a split does.
Where it is genuinely the right answer
Flats where the lease or the freeholder forbids a unit on the elevation. Listed buildings and conservation areas where an external condenser is not going to be permitted. Ground floor positions where a condenser would be within reach of the street. In all of those, a monoblock is not a compromise chosen to save money, it is the only fitted option available.
What you give up
Efficiency, generally, because the whole circuit sits in the conditioned space and the heat rejection path is shorter and less generous. Sound, because the compressor is in the room rather than in the garden. And case size, because everything is in one box, so the head is larger and more visible than a split head of the same capacity.
What the installation involves
Two large diameter cores through an external wall at a set spacing, the wall plate and unit, a condensate route, and an electrical supply. It is closer to a building job than a refrigeration one, which changes who is on site and what certificates you get. The external grilles are permanent and visible, so their position is worth agreeing before the drill comes out.
Questions people ask about wall mounted air conditioner without outdoor unit
Is a unit without an outdoor box the same as a portable?
No. A portable stands on the floor, vents through a flexible hose out of a window and pulls warm air back in around the seal. A monoblock is fixed to the wall, ducted through two proper cored holes, and behaves like a fitted system. They are often confused because both avoid an outdoor unit.
Is it less efficient than a split?
Generally yes, for the same nominal capacity, because the heat rejection path is shorter and the compressor sits inside the cooled space. Whether that matters depends on how much you run it, and it is beside the point where planning or a lease rules out a condenser.
Will it need planning permission?
The unit itself is inside, but the external grilles alter the elevation, and in a conservation area or on a listed building that is exactly the kind of change that needs checking first. Flats also sit outside householder permitted development rights, so the lease and the local planning position both need reading.
Does it still need an engineer?
It needs a competent installer for the coring, the mounting and the electrical circuit, and the electrical work is notifiable. The refrigerant circuit is sealed at the factory, which is what removes the on site charging step, not the need for a proper installation.