ᐅ Converting a Concrete Basement into an Office and Guest Room

We have purchased a semi-detached house and are renovating it diligently. Now, we are considering using part of the basement as an office and guest room. The floor has insulation, with a screed layer on top. The walls are precast concrete panels that have been poured with concrete in the middle.

Two existing basement windows are to be replaced with regular windows. Since the walls are very cold, we are wondering what can be done to avoid high heating costs. There is a radiator connection available, and a small wood-burning stove will be installed.

Are there any recommendations for cost-effective ways to insulate the basement?

Our concrete basement is insulated from the inside with aluminum-foiled expanded polystyrene; I don’t remember the exact specification. The electrical wiring is also installed concealed within the walls.