Historic England and structural engineering guidance both emphasise understanding the existing building before changing it. [1] [2]
Start with what is actually there
Ask the survey to distinguish the main house, rear wings, garages and later additions. Supply earlier drawings and information about removed walls, movement or previous repairs to the engineer. Intrusive investigations may be needed where important structural details are concealed. A date on an estate agent's description does not establish foundation capacity or identify which walls carry loads. [2]
For permitted development, “original” has a specific meaning: the house as it stood on 1 July 1948, or as first built if later. It does not mean the house when you bought it. That distinction can be important for all three of these age groups. [3]
Victorian terraces
Look closely at the relationship between the main house and its rear wing. Historic England identifies surviving service wings, historic plan form and the rhythm of neighbouring rear additions as matters worth preserving or understanding. The rear is not automatically architecturally unimportant just because it is less visible from the street. [1]
Test whether a side-return infill removes features that explain the original plan. Ask for an alternative retaining more of the wing or yard, so that the design choice and its effect on the building can be compared.
Edwardian houses
For an Edwardian property, record the features that actually define the particular house: roof form, bay arrangement, window proportions, brickwork and the relationship to adjacent homes. Ask the designer which of these should lead the extension design, rather than assuming a single period style requires a particular roof or material.
Where historic character is relevant, the proposal should explain how the addition relates to the existing building. Historic England's extension advice supports an approach based on the individual building's significance, scale and materials rather than simply reproducing a fashionable extension type. [4]
1930s semis
For a semi-detached house, review the pair and the street together. Compare a rear option with any side or upper-floor addition using elevations that include the adjoining house. Borough guidance can address roof shape, the apparent balance of additions and the space between buildings; use the guide for the actual council area. [5]
Do not assume an existing garage or earlier extension can carry another storey. The engineer needs to assess the structure and the ground supporting it. [2]
Check heritage and technical requirements separately
A listed building or conservation-area location can change the consents and design evidence required. Those checks sit alongside, rather than replace, building regulations for the extension and structural alterations. [4] [6]
Keep a record of the features to retain, the investigations required and the reasons for the preferred design. This creates a clearer brief than “a modern extension to an old house”.