I would advise that you retain the original skirting boards in any of the original rooms that you have enough of the original skirting for but the new areas and the modern interventions should have a modern approach; I like to use a simple square profile timber skirting and paint it in a satin finish paint to match the colour of the walls. This way the skirting visually merges with the walls, making the room feel more spacious. Also, you are not going into competition with the more detailed original skirting mould.

If you live by the sea ROPE skirting may look good and give a strong identity to the extension. Looks fab in the Solent Forts.

Otherwise, how about being really daring - NO skirting!!! The plaster should be good and strong, so why spoil lovely contemporary lines with a lump of wood???

Good luck if you haven't done it by now.