A half Georgian sash has glazing bars in the upper sash and a single undivided pane in the lower one. The name describes the arrangement rather than the period: it is a Victorian and Edwardian pattern that borrows the look of Georgian glazing for the top half of the window.
The pattern exists because of glass. Georgian windows were divided into small panes because that was the largest size crown and cylinder glass could reliably be made and transported. As sheet glass improved through the nineteenth century, larger panes became affordable, and the obvious move was to open up the lower sash — the one at eye level, where the view matters — while keeping the bars above, where they carry the proportion of the elevation. Half Georgian bar windows are the result, and they are extremely common on terraces of a certain date.
What it tells you about the building
Because the pattern belongs to a fairly narrow window of time, it is a useful dating clue, and a useful check on whether a window is original. A row of houses with half Georgian sashes and one plain two-over-two among them usually means a replacement at some point. Equally, a fully barred Georgian window in a house otherwise of the 1880s may be a later attempt at authenticity rather than the real thing.
Restoration and consent
In a conservation area, and on any listed building, the glazing pattern is one of the details that planning officers look at first, because changing it changes the character of the elevation more than almost anything else. Replacing a half Georgian sash with a plain one, or adding bars to the lower sash to make it "more Georgian", both tend to draw an objection.
Restoring the pattern means matching the bar section, not just the layout. Period glazing bars are slender, and their section is what makes the window read correctly; modern bars are often heavier, and stuck-on bars applied to the face of a sealed unit read as exactly what they are. Where the original bars survive they are repaired in place, and where one has broken it is spliced or replaced to the same profile so that the divisions stay consistent across the elevation.