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1n3r2/q4pbk/3p2pp/r1pPp3/1pB1P3/1P2BP2/2PQ2PP/3R1RK1 w - - 0 1

I was white in the diagram position in an OTB game I played recently. However, I couldn't find a satisfactory plan. Playing f4 is always tricky because it vacates the e5 square for the black knight and opens up the diagonal for the black bishop. I have the feeling I evaluated this position wrong and it is in fact just equal and not better for white. Any thoughts?

I think the f4-plan is doomed to fail. It would be beautiful if we could achieve it in good conditions, however, as you pointed out, f4 is met with ...exf4 followed by probably ...Qe7 and Nd7-e5. We get rid of the pressure on the 'a' file though, so it's better than doing nothing.

If we went for g3, preparing for gxf4 after f4, our effort will be met by ...f5! shattering our dreams.

It would be interesting to try to put our light-squared bishop on a4, but I don't see it working either after 1.Qd3 Nd7 2.Bb5 and now even Nb6 does fine

For those reasons, I think the best answer here is to immediately play c3!? It's true that Black has ...Ra2, but after Qe1 I don't really think it does much. If Black captured on c3, our queen would be now in a great square to fight for the a file. If Black does NOT capture, then we can begin to put some pressure on b4

I think 1. f4 isn't that bad, in this line

1n3r2/q4pbk/3p2pp/r1pPp3/1pB1P3/1P2BP2/2PQ2PP/3R1RK1 w - - 1 0

1. f4 exf4 2. Bxf4 Qe7 3. e5 dxe5 4. d6 Qd8
5. Be3 f5 6. Qd5 Nd7

The white queen is beautifully centered, black has two weaknesses, he's soon to be challenged on the open a-file and after something like 7. Kh1 the white king is more safe and don't forget white has just acquired a passed pawn on the 6th rank.

I don't say by any stretch it's winning, but one can argue it's not a bad plan.

I see a couple ideas:

First your f4 idea seems reasonable. Consider 1.g3 with the idea of f4 when the threat of f5 is good. If Black plays exf4 then gxf4 controls e5. You can even re-deploy the Bishop from c4 to h3 to control the knight.

The second idea, which seems like a clear edge for White to me, is to trade the queen-side pawns by playing c3 and eventually b4. This will expose the d6 pawn as weak while completely destroying Blacks space advantage on that side of the board.

I would play 1.c3

Don't worry about 1...Ra2 that is just a temporary nuisance; simply Qe1 and Black has to deal with the threat of cxb4. On bxc3 2.Qxc3 followed by Ra1 takes over the a-file. Black is suffering here.

Whites position is much better/winning due to double bishop advantage, space advantage and black's poor dark square bishop.

f4 looks like a good plan, however it needs preparation with g3. It threatens f5 with f6 which will completely crush black's position.

c3 looks pretty good as well, breaking up black's queen side.

Of course all of this is my speculation and you should just use an engine to see white's best plan.