to add the tiles are stuck to plasterboard – should I drill in to the tiles OR drill in to the grout lines? And also what type of wall-plug should I use, depending on if it’s tile or grout?

Into the tile, not too near the edge. Standard plastic rawl plug should work if its not a heavy load. Hollow wall anchor if you need a bit more strength

and I think you’re best drilling into the tiles with a metal drill bit.

Usually go with a diamond bit on tiles.

Use a ceramic tile drill bit.

They look like little spear heads and are available for a few quid in most DIY shops.

They won’t crack the tile

As PP said, get the proper drill bits, I got a set of different sized erbauer ones in B&Q for a couple of quid. Just goes through the tiles like butter.

Wall plug based on whatever is behind the tile. Personally I just buy the expensive ones now, Fischer duopower etc. They might be 10x the price, but it’s still only 5p a plug and they tend to work in everything, everytime, rather than just spinning, or finding the plasterboard was too thin to grip.

Depends – what size is the grout line, and what weight is the accessory taking – that will determine the wall plug / screw size?

If it is something like a toilet roll holder, and the grout line is the right size and at the right height I’d use the grout line…(in fact I did). Start the drill slow and make sure the bit is not touching the edge of the tiles. I used yellow fischer plugs and a 5mm bit for my solid wall. You can get specialist plasterboard fittings, but I’d try with a normal plug first, depth of tile, adhesive and plasterboard is going to be 25mm+ which should give plenty of purchase (depending on weight of accessory)

Proper tile bit and a bit of masking tape just to reduce the chances of the bit slipping when you start drilling

If it is something like a toilet roll holder,

Use one of the self-adhesive ones you can get for a few quid on Amazon. They work well.

Use a ceramic tile drill bit.

fair enough!

..and make sure the plug is behind the tile before putting the screw in. if it is actually in the tile, you run the risk of the tile cracking as the plug expands as you tighten the screw

Depends how permanent it is ….do you have spare tiles if you change the fitment later…and even if the tiles are plain white and if you move later filling a hole with grout will not ruin the bathroom

I recently removed a horribly rusted fitting (soap holder thing) and unless I find one with the same length at least some holes will be visible .. I also changed the whole shower (at the same time) which meant pulling the tiles off to redo the plumbing behind and using one of the spares after but the shower rail (height bar thing) still needed mounting and I managed to use an existing hole and cover the grout filled other hole with the bar…

OH hasn’t noticed so I class it as successful….

I did this a while ago – drill the tile. I bought a specific ceramic bit, not like the ones above but circular like the end of a Biro which what looks to be diamond edge. It also came with some plastic contraption to drizzle water into it as you drill.

It worked well. Just take your time (and it should go without saying but for god’s sake turn off the hammer action on your drill!), I put up three shelves which have been strong enough for me to regularly swing on for years when getting out of the shower / bath.