I expanded mine by adding a raised border around the whole thing, about 8 inches high and a foot away from the original border. Actually I did that because it was an "inherited" pond, and it wasn't level - ugly great stretch of pond liner exposed at one end. The new border allowed me to make it level, and the expansion was a bonus.

I put a new liner in, covering the old pond and the new raised border. The the pond became deeper as well as longer and wider... and I got an instant all-round submerged planting shelf where the old border used to be...

Oh, and yes, I had to move everything out and empty the whole thing. I tgink the only way to avoid that is to build a second pond, and then join them - but even that is likely to get messy! Good planning will minimise the time that stuff has to be out of water, or how quick/easy it would be to link a new pond to the old...

Hi Paula, welcome to the forum.
I enlarged my pond by digging another larger/deeper one at the side and 'interconnecting' them together via a 1ft deep x 2ft long canal. I lined the canal and joined it to both liners with double sided liner tape.
This meant the fish stayed in the original pond while the digging was being carried out and I only had to drain the water by about 18" while I 'broke through' and connected them together.
The fish love swimming from one pond to another.
I think it's a good way to keep 'enlarging' - fish aren't stressed by being moved, wildlife isn't disturbed and you save money on liner.
The one 'downside' is that over the years I have had two small leaks in the canal where the liners are joined but it's relatively easy to lower the pond water and patch without bothering the fish.
I want to do the same again and have a third interconnected pond but my husband has put his foot down as the ponds already take up a third of the garden [lol]
Anyone think of a way to bribe him??? (I've tried THAT - didn't work - I gave in!!!)
Sue

Forget the millions of buckets, beg or borrow, a large paddling pool or take a trip to woolies or b&q etc and buy one of the 10ft diameter pools with the inflatable collar. I make your max volume to be 1100 gal, I think the 10ft diameter are quoted at 600 gal but I am fairly sure they hold more, 490 gal per ft of depth. That will save most of your water for the new pond.
Sue you tried what, making him cook??

I built a second pond rather than extending. The second is a raised one with a dry stone wall and other stones around it. I took ages building a waterfall between, but as yet haven't commissioned it.
Having 2 allows me to have one with fish (sticklebacks) and the other fish free.
I don't think adding liner to the existing by sticking it on would be very successful. It is possible to join liners, but I fear it would be likely to leak.

I certainly wouldn't join liners in a 'deep' part of a pond!
But a 'piece' of liner in a 1ft deep canal/stream joined to the 'edge' of liners in 3ft deep ponds is relatively easy to deal with if any leaks occur.
The worse that could happen (if a really BAD leak) is the ponds become separated as the water goes down but the most water I have lost is just a few inches.
I think my small leaks occured because the canal is 'U' shaped and I think the tape may have 'shrunk' and pulled apart slightly in extreme cold weather.
I've now found that by far the best way is to join the liners in the usual way and then put 2 pieces of tape (slightly overlapping) on TOP of the seams, each time leaving the backing paper on the top and rubbing it down very hard with the back of a spoon as you go - then remove the backing paper (leaving the top of the tape uncovered)
This seems to seal the joins really well.

My history of ponds is.....smallish wildlife pond introduced some tiny fish.
Fish grew too large for pond, dug a much larger one at the side and transferred fish.
Introduced Sticklebacks into wildlife pond.
Fish bred and also grew too large for second pond
Drained and enlarged/deepened smaller pond and connected both ponds together.
Dug another 'fish free' wildlife pond.
To many Sticklebacks so transferred some to wildlife pond.
And so it goes on! It never ends [lol]
Sue

Thanks for all the advice folks, it's appreciated.

Think I'm going to try the 'Sue joining the liner method' for starters as it seems less stressful for me and the fish etc. If that doesn't work I'll be off to buy paddling pools!

I'll let you know how I get on and I'll try some before and after photos.

Cheers,

Paula

p.s. think I saw a baby newt yesterday - definitely saw 2 adults about 6 weeks ago