Electric mat or wet system for underfloor heating in new extension?

Most people favour wet UFH systems in new extensions over electric because generally they want to link it into their existing system for convenience or are concerned over the running costs of electric heating. In your case you do not have the option of linking in to your current boiler and both alternatives you are considering are electric. Given this and the fact that it is a new extension I would suggest you go for a more heavy duty 6mm electric UFH cable rather than matting. This is laid on top of the new insulation and covered by the new floor screed in the same way as the wet system but without the added expense of the electric boiler. The reliability of electric UFH has improved considerably over the last few years and if laid correctly and tested before screeding it should serve you well for many years. However if you do decide to go down the electric boiler route just check that your incoming electric mains supply has enough capacity, if it doesn't an upgrade could be very costly. I hope this helps