In the newer Google Maps there is not an option to save a map for offline viewing however the following sneaky tip works around this.
Zoom up to an area of search for it.
Once there, search 'okay maps'
You soon get a notification upon completion.
I first did it zoomed close to my street and it cached in seconds. I then zoomed out so could see the whole of London, it took a minute or two compared to the seconds and now it has stored a lot of it, all streets, for offline viewing in case I'm underground and not viewed that area before. It doesn't cache as much detail as the 3D buildings view. Interestingly though, I tried seeing if zooming out on the whole of the UK and trying the trick would store it all for offline viewing but it states it's too large. Tried London to Brighton but again too large. Seems it'll do all of London at the least, which is great enough. I guess that just means that you can cache maybe cities at a time and still equate to having a lot of map data stored for offline. Maps cache size is only taking up 37.2mb too which is nice.
Hope that's of use!
Credits go to this original source: