Our neighbour’s extension/adjacent property is too imposing or has distant windows which overlook.
If the issue is primarily the unsightly nature and imposition, then a mixed planting of upright evergreen and deciduous trees, as described for Problem #1, can be an attractive solution. Alternatively, a single large evergreen specimen can break up the space and divert the eye.
For medium spaces try Magnolia grandiflora ‘Gallisonniere’ or ‘Goliath’, or for more expansive screening, try planting a row of repeating screening evergreens with a selection of deciduous trees in front for contrast and seasonal interest.
For smaller spaces try evergreens which can be pruned to keep in check, such as Prunus lusitanica, Prunus lusitanica ‘Myrtifolia’, Ligustrum lucidum ‘Excelsum Superbum’, Photinia x fraserii ‘Red Robin’ or Ilex ‘Nellie R. Stevens’. For something a bit more unusual, you might try the highly fragrant try Osmanthus x fortunei, or Camellia japonica in a tree form.
Alternatively, densely branching deciduous trees such as Prunus cerasifera ‘Nigra’, Carpinus betulus ‘Fastigiata’, Acer platanoides ‘Globosum’, or Pyrus x calleryana ‘Chanticleer’ can also be effective at breaking up a blank wall whilst still providing seasonal interest. Choose foliage colours which complement or contrast the brickwork and suddenly the offending wall becomes a canvas. For example, dark green foliage contrasts beautifully with orange brick, and red, purple, and jade tones are stunning against gray masonry. White stemmed birch are exquisite against very dark timber or brick.
If there is a definite year-round privacy issue, such as a window (or windows) with an outlook directly into the bedroom, kitchen, etc. then see solutions for Problem #1 or #3, or #4.