See how a spacer changes your effective ET and where the wheel sits relative to the hub.
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Wheel offset (ET) is the distance in millimetres from the wheel's centreline to its mounting face. A higher ET pushes the wheel inward; a lower ET moves it outward. On a BMW E36, the stock front hubs run ET41 (sedan/coupe) with a 5×120 bolt pattern. Rear ET is 20 on most variants.
A spacer reduces the effective ET by exactly its thickness. Fitting a 20 mm spacer on a front wheel with ET41 gives an effective ET of 21 — 20 mm more poke than stock. This also increases the scrub radius, which adds steering feedback (and load) through the hub and wheel bearing.
The outer limit is your arch and fender. Most E36 sedans can run 20–25 mm rear spacers before rubbing at full compression. On the front, arch clearance is tighter due to the strut; 10–15 mm is usually the practical maximum before you need rolled fenders. Always use hub-centric spacers and the correct seat type (ball seat for most E36 aftermarket wheels, conical for OEM).