Plan the layout first
Place the furniture you want to keep on a floor plan, then add the new space. This avoids gaining square metres while the seating or dining area still feels awkward.
The existing rear wall may be load-bearing. A wider opening therefore needs structural design and suitable temporary support during construction.
Bring light deeper into the house
An extension can make the existing living room darker. Discuss glazing positions and rooflights so daylight also reaches the space behind the extension.
The existing living room remains part of the design
Consider the layout from front door to garden. Where do shoes go, where do visitors walk and what can you see from the sofa? An extension may free up existing space for play or work. Include sockets, wall lights and curtains, then decide the final external dimensions. That way, you build for usable living space rather than the largest number on a drawing.
Let furniture guide the dimensions
Mark the table and chairs on the floor and walk past with chairs pulled out. Do the same for an island, then position the new facade. Extra floor along a solid wall may be more useful than the same area beside a busy walkway. Check television placement against sunlight and reflections.