Same beans and same recipe taste flat at home and excellent at a friend's, is it really the water?

It is the water, and the reason a jug filter did nothing is the most useful part of the answer.

Coffee is about ninety-eight percent water, and the minerals in it are part of the extraction rather than a background. Two matter:

Magnesium and calcium actively bind to flavour compounds and pull them out of the grounds. Too little and the brew is thin and flat - nothing is being extracted efficiently. This is the empty-tasting failure.

Bicarbonate buffers acidity. Too much and it neutralises the fruit and brightness, and everything tastes dull and slightly chalky. Hard water is typically high here, and this is almost certainly your problem: it is not that your water is dirty, it is that it is chemically flattening the cup.

Why the jug filter did not help: most of them are ion-exchange or carbon. They remove chlorine, some metals and some taste, and they reduce hardness modestly at best. They are designed to protect your kettle and improve tea, not to hit a brewing target. Filtering hard water usually gives you slightly softer hard water.

Why the bottled water worked: you happened to buy something in a much better range. Which is also why bottled water is inconsistent advice: the brands vary enormously and some are worse than tap.

Taking your own beans and grinder to a friend's flat is a genuinely good experiment and almost nobody does it.

The practical options, from cheapest to most involved.

Read the label on bottled water and pick deliberately. Every bottle states its mineral content. You are looking for moderate calcium and magnesium and, critically, low bicarbonate. Once you find one that works, it is repeatable and it costs what it costs. This is the lowest-effort real fix and worth doing once just to confirm the diagnosis.

Dilute your tap water with distilled or deionised water. Half and half is a reasonable starting point in a hard area. Cheap, and it moves both hardness and bicarbonate in the right direction at once.

Mineral concentrates. Small sachets or drops you add to distilled water to hit a target composition. Consistent and controllable, and about as much faff as it sounds. Genuinely worth it if you are chasing the last ten percent; overkill for most people.

A proper filter, not a jug. Some under-sink and in-line filters do reduce bicarbonate meaningfully. Worth investigating if you also have an espresso machine, because hard water is destroying that as well.

What I would actually do in your position: buy three different bottled waters, brew the same beans with each, and see how large the difference is. That tells you how much this is worth to you before you spend anything.

One caution before you go down this road, and a note on the other end of it.

Do not brew with distilled or fully softened water on its own. Water with essentially no minerals extracts badly, tastes hollow and flat, and is corrosive to some equipment. Softeners that work by ion exchange swap hardness for sodium, which does not help extraction at all - so a house with a water softener can produce worse coffee than one without, and people are surprised by that.

The target is not soft, it is balanced.

And the honest ordering of what matters in a cup, since water is fashionable at the moment:

1. The beans, and how fresh they are
2. The grind and the recipe
3. The water
4. Everything else

Water is a real and often overlooked variable, and it is not going to rescue stale beans or a bad grinder. Your case is a good one specifically because you controlled everything else and moved location: that is a clean experiment and it points at the water unambiguously.

Also worth doing while you are here: descale your kettle. In a hard area it will be furred, and a scaled kettle contributes its own flavour on top of everything above.