While there had been a huge increase in the population of the rural counties there was a greater shift in the population from the impoverished land to the large towns and London. At the same time, society was restructuring, with the labouring classes dividing into artisans and labourers. The cities offered labourers housing in tenement blocks, rookeries and lodging houses, and philanthropic societies turned their attention towards them. The rural "Labourers’ Friend Society expanded in 1844, and as a result of the various reports on the housing conditions of the urban working classes, it was reconstituted as the "Society for Improving the Condition of the Labouring Classes".  The earlier designs they published had been for semi-detached dwellings but the first properties they built were tenements and lodging houses. In their 1850 publication 'The Dwellings of the Labouring Classes', written by Henry Roberts, there were plans for model 'semi-detached'cottages for workers in towns and the city. In 1866 the Metropolitan Association for Improving the Dwellings of the Industrious Classes, founded by Rev Henry Taylor, built Alexander Cottages at Beckenham in Kent, on land provided by the Duke of Westminster. This development initially comprised 16 pairs of semis but two years later they had built 164 semis. 

In Birmingham, Wolverhampton and the Potteries there was a tradition dating from the 1790s of artisans saving through mutuals and Friendly Societies. In the 1840s, the permanent building society model was adopted. The Woolwich Equitable was founded in 1847, the Leeds Permanent in 1848 and Bradford Equitable in 1851. Artisans could invest and then borrow a sum for a mortgage on their own property.

In the wool towns of Yorkshire three families built villages for their workers. In each, there was an hierarchy of houses: houses in long terraces for the worker, larger houses in shorter terraces for the overlookers, semi-detached houses for the junior managers, and detached houses for the elite.  The first village was built by Colonel Edward Ackroyd, at Copley, West Yorkshire between 1849–53, the second by Sir Titus Salt, between 1851 and 1861 at Saltaire, and the third was the West Hill Park Estate in Halifax built by John Crossley. The model villages in Lancashire came later, with developments like Houldsworth Village. Semi detached housing in colliery villages was rare; status here was determined by the length of the terrace.

The development of Port Sunlight and Bournville was important. Port Sunlight model village commenced in 1887. William Lever used architects William Owen and his son Segar Owen and stated in 1888 that:

"It is my and my brother’s hope, some day, to build houses in which our work-people will be able to live and be comfortable – semi-detached houses with gardens back and front, in which they will be able to know more about the science of life than they can in a back-to-back slum

At Bournville in 1879 the Cadbury development started with a detached house for the manager and six pairs of semis with large gardens for key workers. As it expanded after 1895 the village was made up of semis and short terraces. Cadbury fulfilled the dreams of the Society for Improving the Condition of the Labouring Classes by proving that: "a low density layout could be a practical possibility even for the working classes, and unwittingly he opened the flood gates to a new kind of suburbia " The examples of Bournville, and Port Sunlight were seized by Ebenezer Howard and became key models for the Garden City movement

The middle class became an important and expanding group in the nineteenth century. With industrialisation came material gain to the capitalist entrepreneur. New professions came into existence to serve their needs; insurers, engineers, designers. The growth in the population required more architects, lawyers, teachers, doctors, dentists and shopkeepers. There emerged hierarchical tiers within the middle class, each watching each other's status. The baseline appears to have been an income of 150.00.00 pa., as stated in A New system of Practical Domestic Economy (1820-1840). In 1851, it is estimated that out of a total population of 18,000,000, 3.000.000 would have been considered to be middle class. 

Built circa 1870 two semi-detached cottages at Mentmore, UK masquerade as one Mock Tudor style house.
Semi-detached houses first began to be planned systematically in late 18th-century Georgian architecture, as a suburban compromise between the terraced houses of close to the city centre, and the detached "villas" further out, where land was cheaper. There had been occasional examples in town centres going back to medieval times. Most early examples are in what are now the outer fringes of Central London, but were then in areas being built up for the first time. Blackheath, Chalk Farm and St John's Wood are among the areas contesting being the original home of the semi. Sir John Summerson gave primacy to the Eyre Estate of St John's Wood. A plan for this exists dated 1794, where "the whole development consists of pairs of semi-detached houses, So far as I know, this is the first recorded scheme of the kind". The French Wars put an end to this scheme, but when the development was finally built it retained the semi-detached form, "a revolution of striking significance and far-reaching effect".

A particular style of semi seen from these early years is the row of houses where each pair is linked by a wall along the frontage, as at The Paragon in Blackheath, where a blank colonnade runs between the houses. Most early examples were relatively large houses with access at the rear, but from around the same time, rural cottages were sometimes built as "double cottages", mainly to save the expense of extra walls.

During the 19th century, a father and son architectural partnership, John Shaw, Sr. and John Shaw, Jr., drew up designs for semi-detached housing in London. Examples of their work can be seen in Chalk Farm, North London. John Nash better known for his regency terraces built some semi-detached villas either side of the Regent's Canal, they were styled to appear as substantial single detached villas with the entrances to the side. Similarly John Claudius Loudon, the landscape gardener built a pair of semi-detached villas in Porchester Terrace in 1825, fashioned to appear as a single house. In his 1838 book, the Suburban Gardener and Villa Companion, he gives advice on how to disguise the join by using false windows.