Does bottle conditioning effect the ABV?

AFAIK, maybe a tiny bit. But most of the alcohol is produced during primary, which is where the yeast does its heavy lifting. Anything in secondary or bottle is mostly flavor. Once the sugars are gone, there's nothing left for the yeast to eat. Or, once the yeast is tired, it can't eat more sugar. YMMV.

 

If by bottle conditioning you mean that carbonation is achieved through refermentation in the bottle, then the answer is yes.

How much depends on how much sugar is added at bottling to achieve desired carbonation levels.

 

For brett beers such as Orval that are expected to develop further in the bottle, they must give the brett some sugar to eat. Is the marked ABV just a ballpark target then?

 

There is a small increase in the abv due to refermentation for carbonation. If you open a bottle that is a gusher, then more sugar was added by the brewer than what was needed, although some beer styles have a high level of carbonation by design (just not to the level of being a gusher).

The abv on the label is an estimate by the brewer of the correct number. The TTB recognizes that brewing is not an exact science and that batches can vary, so some latitude is given on what gets printed. I think it is +/- .2 or .3 points.

I'm going to guess that a normal increase in the abv that is caused by the refermentation would be only like a 0.1 to 0.2 percentage point increase.

 

I think the Brett just eats the complex sugar that the normal yeast can't eat during the primary. I don't think additional sugar is added for the brett.

 

I don't suspect the difference will be anything worth making a note of because the gravity the brewery is looking for will have been consistent. I'd be more worried about a fruit infused beer than one which is bottle conditioned for a problem to be wary of.

 

The yeast should have done their job within 2 weeks after bottling which likely has gone by before you purchase the bottle. So it doesn't matter whether you keep the bottle at room temp or chilled.

However, that beer also benefits by maturing (also sometimes referred to as conditioning or aging) at cellar temp which is generally considered to be around 50 degrees.