Alleys and rear lanes are important parts of accessible urbanism because they get a lot of messy things off the street, enhancing Walk Appeal, but if you don't get the geometries right, they can fail spectacularly. Planners of new communities should know what they're doing and get all this right, but that's not always the case. The more likely failure is in existing places that are being retrofitted with alleys and rear lanes because in those places, decision-makers may be dealing with alleys and rear lanes for the first time, unaware of all the places they can get snake-bitten in spite of good intentions. The following guidelines have been used widely in the US with a track record dating back to the reintroduction of rear lanes around 1990.

Types

It's important to understand the differences between alleys and rear lanes. The alley is a more urban condition occurring mostly in town centers. It is usually paved edge-to-edge and buildings backing onto the alley have zero setback. The rear lane is a less urban condition serving mostly- or entirely-residential streets. It should have a narrower right-of-way than the alley, and is paved in the middle with a single lane that is narrower than the right-of-way. The much rarer (in the US) mews court shares some conditions with both alleys and rear lanes. The mews court is paved edge-to-edge like an alley, but serves mostly residential uses, bounded either with single-story attached garages or more preferably by 2-3 story mews units that may have either an apartment or a home workplace above.

Dimensions

The right dimensions are essential to getting the geometries right. All three (alleys, rear lanes, and mews courts) should place garages, carriage houses, carports, mews units, and parking spaces with 30 feet between those on the other side to allow room for large vehicles to turn into them. And because alleys and mews courts have zero setback, the right-of-way for each is also 30 feet. This dimension is the primary requirement for alleys and mews courts, but rear lanes have additional requirements.

Rear Lanes

Some municipalities require rear lanes to be paved for the full width of the right-of-way, but this is overkill. It is "a lane," after all, not a two-lane street. And some municipalities require a right-of-way similar to that of a street, but a 20 foot rear lane right-of-way is ample. A 5' rear lane setback produces a garage-to-garage distance of 5 feet + 20 feet + 5 feet = 30 feet garage-to-garage as noted above. The problem occurs when the garage is located slightly more than 5 feet from the rear property line because it may look like you can pull a car in behind the garage and not be in the rear, but the rear end of your car can be in the lane right-of-way, and some vehicles with large turning radii need the full 20 foot right-of-way plus their 5 feet garage setback to make the turn. So I use a garage setback of exactly 5 feet to leave no doubt you can't pull in behind the garage or 15 feet minimum. Technically, it should be 18 feet minimum since that's the depth of a parking space, but in my 20+ years of planning and 25+ years of implementing other people's plans I've never seen the perfect storm where the 15 foot minimum doesn't work. But what about the situations where a large vehicle needs the full right-of-way to make the turn in to their garage, and only 12 feet of the right-of-way is paved? I've long allowed for the person with the big turning radius vehicle to replace the grass on the other side of the alley with Grassblock or similar at their own expense.

Other Setbacks

It is essential to understand that all setback dimensions are the same whether vehicles are parked in garages, carriage houses, carports, mews units, or a parking space which must be deep enough so the vehicle can be parked completely off the right-of-way. Other outbuildings should follow the same setbacks unless they're built of loadbearing brick walls, which are noncombustible. Fences, hedges, and landscape walls can be built right to all property lines, but I recommend setting back an inch or so from adjoining neighbors' property lines to avoid disputes with a future property owner even if the current one is completely happy with the fence.

Neckdowns

A 30+ foot hole in the side street streetscape is large, so it's best to "neck down" the ends of alleys, rear lanes, and mews courts to the minimum your city will allow. Back Bay in Boston necks down alleys to little more than the alley width of 8 feet with building wings that leave just enough room for a sidewalk into the alley, and Philadelphia does similar things. London's solution is leaner and less expensive than building wings, but more striking: entries to mews courts there are often through a portal that looks like a small triumphal arch.

All of the geometries and dimensions noted here are such second nature to me after all these years working with them, but please let me know if something's really unclear and I might come back and add some diagrams where needed... thanks!