These go in before the liner does, between the bare ground and the vinyl. Once the pool is full of water they cannot be added without draining it and lifting the liner back out — which is why they sit next to the liner rather than with the other add-ons.

Pool cove

A foam wedge that fills the sharp angle where the wall meets the floor. The weight of the water pushes the liner hard into that corner, and the cove gives it a curve to sit against instead of a right angle — so the vinyl is supported at the point it takes the most strain. Sold by the strip, so a pool this size needs several to go all the way around.

Floor pad

A cushioned layer between the ground and the liner. It stops small stones, grit and roots working through the vinyl from underneath, and it is noticeably softer underfoot.

Coping / receiver clips

The plastic pieces that hold the top edge of the liner onto the wall. Your pool kit includes what it needs — these are here as spares or replacements, and are also sold singly.

If you skip them your pool will still fill and hold water. The risk comes later: a puncture from below, or stress where the wall meets the floor, usually shows up as a slow leak you have to hunt for. Fixing it means draining the pool and patching or replacing the liner — a far bigger job than fitting the pad would have been.

On a smooth, stone-free sand base a floor pad matters less. A cove is used on nearly every above-ground build.

A pump pulls water out of the pool through the skimmer, pushes it through a filter that traps the dirt, and sends it back in clean. It runs a few hours a day all season. Without one the water goes cloudy and then green within days — chemicals alone will not keep it clear.

Choosing between them comes down to two numbers, and both of them buy you less work rather than a better swim:

Tank size — 16", 19", 22"

How much sand is inside. A bigger tank holds more dirt before it needs rinsing out, so you backwash less often and the filter works at lower pressure. On a large pool a small tank is the thing you end up regretting.

Pump horsepower — 1 HP, 2 HP

How fast it moves the water. Aim to push the whole pool through the filter roughly twice a day. Bigger pools need more, and so does anything else you plumb in later such as a heater or a pressure-side cleaner.

These are all sand filters, which is the usual choice for an above-ground pool: you rinse the sand clean in place by reversing the flow for a minute or two, rather than buying replacement cartridges, and the sand itself lasts for years.

If you are torn, size up. An oversized filter costs a little more once and saves you backwashing all summer. Already own a pump and filter? Use the skip option in this step.

Your pool wall is taller than most people can climb, so a ladder is how anyone actually gets in and out safely. The choice is decided almost entirely by one question: is there a deck alongside your pool?

No deck — choose an A-frame

It straddles the wall with steps on the outside and the inside, so you climb up from the ground and back down into the water. This is what most above-ground pools need.

You have a deck — choose an in-pool step

Only the inside half exists. You walk out along the deck and step down into the pool. It has no outside steps at all, so it cannot be used on a pool without a deck.

Two other things worth checking before you pick:

• Wall height. Match the ladder to your wall, not to the pool's width.
• Keeping children out. Many A-frames have an outer section that swings up and locks, or lifts off entirely, so the pool cannot be climbed into while nobody is watching.

Buying a deck step for a pool with no deck is the most common mistake here — there is no way to reach the top of the wall with it.