I think the best piece of advice I could give any one thinking of building a koi pond is join your local koi club 

You can ask other members how they've built their ponds, what problems they've encountered, what filters they've used. By doing this you'll build up a picture of what works and more importantly what doesn't, it can save you thousands of pounds and weeks of wasted time. Another good thing about being a member of a koi club is that through various club events you'll get to see a lot of different designs for ponds. It was on one of these visits to a fellow club member's pond that I saw the inspiration for my design.  This pond was a long rectangle design, nothing unusual about that apart from ½ the pond was covered by a conservatory. Which I thought was a brilliant idea, you can sit outside is the summer watching your koi and then in winter when a traditional pond would have the covers fitted you can sit indoors enjoying you koi. Also for 4 months a year I go to work in the dark and come home in the dark so having somewhere warm to sit and unwind from work sounded great.

I had to adapt the design a little because my garden isn't a nice rectangle, more a square with a large corner cut off, also because of where the drainage, electricity supply, water supply and garden gate where I ended up with an L-shaped design, this wasn't my preferred shape as I'd been warned by pond owners with L shaped ponds that the ‘L' hampers the flow of water though the pond and it's a bugger to catch your koi !. Never the less after about 10 different designs it's what I ended up with.

I cleared the old 6ft circular pond out of the way, keeping the koi in a vat at the bottom of the garden. I hooked up the original gravity feed filters, modifying the pipe work to run pump fed. I didn't hook up the UV and after a few weeks I had a nice green soup at the bottom of the garden, the koi didn't seam to mind. I also covered the vat with a net to stop the koi jumping out and stop anything else getting in.

The pond would be a raised design sitting on a ring beam of concrete. The conservatory would then sit on top of the pond walls. Foundations were dug 1 ft wide by 1 ft deep and the 10 mm rods reinforcing rods where added and then the concrete poured in. I hired a cement lorry that mixes the concrete as you use it, very good, you just need to be well prepared and have at least 3 wheelbarrows on the go (with volunteers) to keep up with the mixer. The advantage of using this is that it's a lot quicker than mixing it by hand or with a DIY cement mixer, and unlike the conventional cement mixers where you have to guess the amount you require , with this system the operator just pulls a lever and  hey presto ready mixed concrete. Took about an hour to complete the job.

Next job was to dig out the pond, for this I hired a mini digger, the plan being to drive in down the alley and though the gate, however although it would fit down the alley and though a normal gate it didn't have the space to do the 90 degree turn required to get though the back gate. I nearly said "it won't fit, so I dig it out by hand" I'm so glad I didn't! I ended up taking down the fence to get the digger in the garden. Once we got started it was a joy to use. I will never dig out a pond by hand again! I'd just hired the digger for the day and spent an hour getting it into the garden but we completely dug the pond by three o'clock. The waste soil had filled the drive to a depth of 4ft, we hired a grab lorry to take away the spoil, again a lot easier to using skips.

The next day we tidied up the walls and then worked out exactly were the bottom drain was going, this was then dug out by hand, the soil was thick clay and it took almost as long to dig the trench and break thought under the ring beam as it did to dig the entire pond out using the digger, next time I hire the digger for two days !

Next little job was to build the walls, constructed using 9 inch blocks laid on their side, the curves in the walls caused a lot of work, we had to cut the blocks in half using a grinder to score the blocks and then a hammer and chisel to split them. After doing the first run using this method I borrowed a hydraulic block splitter from one of the builders we us at work, another enlightening experience, my son and I cut all the blocks effortlessly in a few hours, I'd recommend hiring one, no noise, no dust, no hammering.  Just stick the block in the jaws, pump the handle a few times and you end up with two blocks both with a nice clean cut.

The bottom drain was cemented in place and then the filters fitted. We then fitted the liner. We had loads of problems, you're supposed to spread out the liner on a flat piece of ground and then drop it in to place ensuring there as few folds as possible. We didn't have any space to do this so needless to say it wasn't plain sailing, but we got there eventually.
I not entirely happy with the end result but given the shape of the pond I don't think we could of got it any better.
Next task was to cut the hole in the liner for the bottom drain. I made a mistake by not marking the clamping ring that secures the liner to the bottom drain. The bottom drain is fibre glass and the ring had been drilled to suit, so it only fitted one way. Wouldn't normally be a problem as you'd just rotate the ring until the holes line up, however when your 5 ½ foot down a hole, the bottom drain was black, the liner is black, the sealant I was using was..... you guessed it  black and by the time I'd finished I'd got sealant everywhere and I was black. After the first fill I noticed the level was dropping so I had to do the whole thing over again!.

Once the pond was sealed I filled the pond up with a water meter, 12672 litres or 2816 gallons of water , the old pond was 300gallon so the koi should be happy in their new home. I used slate to finish the walls, but inevitably as you seat the slabs on the wall you'll drop cement in the water, so the whole lot has to be drained and cleaned again.
The conservatory could now be built over the smaller part of the L section, must admit I wasn't impressed with the quality of the parts and it was a pig to assemble. 

A friend came round a rendered the walls for me, did an excellent job.

The pond had been filled for a week, no sign of leak, filters and electrics where finished, so it was time to add the koi. The water was the pumped out of the vat into the pond. The filter media from the old filter was added to the new filter to maintain the water quality and kick start the new filters. The koi where then added, they swam away quite happy, didn't lose any during the build which was nice. Because I used the old filter media and had the same amount of koi the pond didn't suffer any water quality issues, however I took it easy on the food and didn't add any new fish during 2005. The main pond was finished in 2005, during 2006 I did some more work on the conservatory but most of my time has been spent on the house. The plan for 2007 is to finish the conservatroy, landscaping and decking. Once that's done I can truly relax and enjoy my koi.