Start by confirming it is coolant
Owners describe the same early signs across most engines: a low coolant warning on the dash, a sweet smell after driving, and pink or blue crust around hoses, seams and flanges. A coolant puddle under the front of the car is the obvious one, but several of the leaks below show up as crust or residue first and as a puddle only later.
Two other symptoms are worth separating out. Coolant loss with no visible leak points toward something internal, such as the cylinder head gasket or, on diesels, the EGR cooler. And a sweet smell inside the cabin with foggy windows or a wet passenger footwell points at the heater core rather than anything under the hood. Oil leaks are also often mistaken for a head gasket problem, so confirm what fluid you are looking at before authorizing work.
N52, N54 and N55 inline-sixes
On these engines the classic coolant failure is the electric water pump, usually replaced together with the thermostat because they share the same drain-down and access. Owners report overheating warnings, the fan running at full speed and the temperature climbing in traffic, and pump fault codes in the 2E81 to 2E85 range. See Electric Water Pump + Thermostat for what owners were quoted, and Thermostat Replacement if only the thermostat has failed.
On F30 3 Series and F32 4 Series cars the coolant expansion tank is the other repeat offender. One N55 owner in our data went through three tanks in eight years, each failing at the bonded seam. A cracked tank vent hose causes the same sudden leak and is a much cheaper fix, and on early N20 and N26 cars built up to August 2012 a BMW bulletin says a corroded level-sensor connector can trigger a false low coolant warning, so rule that out before buying a tank. The details are on F30 Coolant Expansion Tank & Coolant Lines.
Two older-chassis causes are worth knowing about if you are shopping for an E9x: leaking radiator end tanks (Radiator Replacement (E9x)) and the heater core behind the dash (Heater Core Replacement), which is labor-heavy because the dash area has to be opened up. On the N54 and N55, also remember that the oil filter housing gasket leak (Oil Filter Housing Gasket (OFHG)) can in rare cases involve the oil cooler gasket and put oil and coolant in the same place.
N20, B46 and B48 four-cylinders
The N20 uses an electric pump, while the B46 and B48 use a mechanical pump with a bracket. On the B48 a BMW technician in our source recommends replacing the full pump and bracket assembly because the bracket gasket can leak, while another says to replace only the cracked part. Either way, a leak at the front of the engine is the first place to look. The repair page is Water Pump (N20 / B46 / B48).
The other known B46/B48 source is the turbo coolant return line, which usually leaks at its rubber section or its connections. Owners notice slow coolant loss and crusty residue around the turbo lines. The feed line sits on top and has to come out first, and several cups of coolant spill when the return line comes off at the thermostat end. See B46/B48 Turbo Coolant Return Line.
For a persistent unexplained coolant loss on an N20, the head gasket is the scary possibility. Our data has only a couple of quotes and they differ wildly in scope, so treat Head Gasket Replacement as a rough guide, and get a combustion gas or leak-down test first, because oil leaks are often mistaken for a head gasket failure.
B58 inline-six (and the 330i B48)
The B58 and some B48 cars have a plastic oil filter housing that cracks and leaks coolant or oil. Owners describe a coolant puddle, a low coolant warning, a smell from the rear right of the engine, and in some cases the housing cracking while the oil filter cap is being removed. Book time quoted in these threads is about five hours, but some shops billed far more, and many quotes also add the heat management module, hoses or the water pump, so compare like for like. See B58 / B48 Oil Filter Housing (Coolant Leak).
The B58 water pump is the second cause. Early production engines may need the pump support or housing replaced along with the pump because of a design change, and that scope question is what separates a cheap quote from an expensive one. Ask up front whether your build date needs the support. Details are on B58 Water Pump (Coolant Pump).
N63 V8 and the diesels
On the twin-turbo N63 V8 the coolant lines sit in the hot-V under the turbos. Owners see a low coolant warning, a coolant smell or steam from the engine valley, and residue on top of the engine. BMW's fix uses updated metal lines, and the official procedure removes both turbos. Ask whether a quote covers all the lines under the turbos or only the accessible upper hoses, and check for open campaigns by VIN, because owners in the source thread got goodwill discounts by asking. See N63 Turbo Coolant Lines.
On the X5 35d and 335d diesels, a P2457 EGR cooling performance code with coolant loss can mean a failed EGR cooler. Some owners cleared it with a bypass-flap spring fix instead of a new cooler, and 2014 to 2017 X5 35d models fall under a BMW recall, so check eligibility by VIN first. See X5 35d EGR Cooler (Diesel).
After the repair: coolant service and scope
Once any cooling component has been replaced, the system has to be refilled and bled, so many owners do a coolant service at the same time. On N55, N20 and B58 cars the coolant gets replaced anyway when the water pump or thermostat is done. If you are getting a standalone coolant service, ask whether the price includes a cooling-system pressure test. See Coolant Flush / Coolant Service.
The consistent advice across these repair pages is to ask for an itemized quote. The same leak can be quoted as a single hose or as a pump, thermostat, hoses and coolant package, and that scope difference is usually the reason two quotes for the same symptom look nothing alike.