If you mean the sides which are against the sheds are going brown, there's not much you can do about that. Otherwise, if they're browning all over, drought might be an issue - you haven't said what conifers they are, but cypress aphid might be a problem.

Reflected heat would be a problem here, but I don't know how much of one it would be in the UK. Digging for foundations may also be a factor, if it cut major roots.