The latest figures on Perth's office space has revealed about a fifth is vacant, the equivalent of 21.5 fully vacant 240 St Georges Terrace buildings.

But Perth's office space "peak vacancy" might be in the past, according to the boss of a property research firm conducting the analysis.

Almost 21 per cent of Perth's office space is vacant, the equivalent of 21.5 vacant 240 St Georges Terrace office blocks.

Y Research's Damian Stone said demand from tenants and the imminent end of an office supply wave meant the market should be looking up.

Mr Stone analysed 2500 office properties across the city, totalling about 4.1 million square metres of space.