Why Are House Prices So High In Cornwall, When Salaries Are So Low?

Limited land.
Popular place (warmest weather in GB)

The second house phenomenon gets blamed a lot; Londoners come in with a lot of money and can pay relatively a lot for a second home.  The official statistic is that 9.9% of homes in Cornwall are owned as second homes, let out for profit or just shut up most of the year.  

But Cornwall's county website says that about 16,000 (just 6.9%) of the 232,000 residential properties in Cornwall are vacant or kept as second homes.

So I'm not sure the "rich Londoner with a second home" should really be blamed.  Especially as something like 27% of homes in London itself are second homes.  10% of homes in North Norfolk, where I live, are second homes, and we don't have such high property prices.

The more obvious reason could be... lack of new build home and renovations of derelict property as people immigrate in and existing residents want to move out from Mum+Dad's.

Cornwall measures about 1382 sq miles.  Norfolk measures 2050 sq miles.  Did a quick search on Primelocation: Cornwall only had 116 newbuild properties, Norfolk has far over 200.  So Norfolk has more room to grow, and lots more being built.  Cornwall is choking on its own popularity, lack of suitable places to build, and inability to keep up.