Apparently identical buildings may require different foundations on the same site, due to variations in geology or previous uses of the site, or perhaps the infilling of ditches, ponds or quarry workings and clay extraction pits.
Masonry movement joints may be required in different locations. This is because elevations with southerly and westerly aspects are subject to large variations in temperature and therefore significant expansion and contraction of masonry, or drying and wetting of subsoil. The latter may also be affected further by trees near to a building.
A Civil Engineer can do for a penny what any fool can do for a pound
If you are considering commissioning a Structural Engineer, bear in mind that design is a creative process which is not necesssarily easily measured by time - one additional hour of design may lead to the saving of hundreds or thousands of pounds in material and construction cost, or yield a design of greater aesthetic or functional beauty. A fixed, low-as-possible fee is less likely to produce an economic, elegant design.