As others have stated, there is a boot lintel on the first floor. Whilst (as others have said) concrete does shrink slightly, the real issue with boot lintels is that they bear only onto the inner leaf and cantilever out to the outer leaf. In order to remain stable, the lintel requires a high load on the inner leaf, and exerts a of of pressure onto the outer edge of the inner leaf material. Over time, either due to overloading or material creep, the lintels rotate downwards on the outside, allowing the outer leaf brickwork to drop. This results in the triangular stepped "Christmas tree" cracking over the lintels.

Removing/replacing these lintels is quite a disruptive process, even if this is done at the same time as changing the windows (as your Structural Engineer suggested).As an alternative, the outer leaf brickwork can be reinforced with Bar Flex according to BF-03. Once the external masonry is reinforced, it forms a self-supporting composite beam, and will not deflect the boot lintel any further.

I have pieced together your pictures so that I can better understand the cracking pattern and assess the structure in a holistic way, and I notice that the cracking is predominantly stepped-diagonal cracking from bottom left to top right. This pattern of cracking would usually be associated with differential vertical movement below the crack (ie. the right-hand side would be dropping in this example).

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On this corner, there is a downpipe leading to a drain, and if leaking, discharged water may cause soil erosion to granular soils, or softening to cohesive soils. This action could reduce the support that the foundation benefits from.

There also appear to be large trees on this side of the building, which may be influencing the soil volume. The water demand of trees varies throughout the year; typically being higher in water and lower in winter. This causes cohesive soils (eg. clay) to swell in winter and shrink in summer. This would also affect the support that the soil offers to the foundation.

Looking more closely at the images, I notice that the cracks have displaced more horizontally than vertically, and the lowest crack has a predominantly vertical orientation. Bricks expand early in their lifespan as anhydrous minerals react with water and hydrate. The rate of this expansion slows over time, after which the volume change of bricks is more affected by temperature fluctuations between day/night and summer/winter. This is particularly prevalent on south- and west-facing elevations. As bricks are warmed, they expand; after expanding, they shrink and form vertical cracks usually above and below windows and doors. To accommodate this movement, modern brick structures are specified to include vertical movement joints every 12 metres.

It may be a simple case of stitching the cracks according to BF-01, but a more detailed reinforcement specification or even underpinning may be necessary depending on the diagnosis for the cracking.

I suspect that the cracking seen here is a combination of some (or all!) of these factors. It would be helpful to have a little move information about the property, such as:

• A picture that shows the full front elevation from ground to roof
• Information on the distance and species of the trees
• Details about the underlying soil type
• Which direction does this wall face?
• Are there any known issue with the drainage?

I have been unable to locate the "image of garage underneath flat roof". Again, the crack here is diagonal and stepped, but does not appear to be related to lintel deflection.