This paper explores the potential role of the incremental planning of speech in interfering with the conditioning of the variable contraction of English is. Previous research has found that a variable alternation which is conditioned by the nature of the element that follows it can have this conditioning disrupted when a speaker fails to plan what that following element will be (Wagner 2011, Tanner et al. 2015). The strength of the effect of that following element on the variable alternation thus diminishes the less likely advance planning is. I extend this research, which has so far only examined following phonological elements, to look at whether this finding holds when a following element effect is localized in the syntactic domain. Taking is-contraction as my dependent variable, I first provide a detailed account of the role of following constituent category in conditioning this variable, documenting a robust effect in Mainstream American English with a hierarchy of environments very similar to what has been found in studies of the contraction and deletion of is in African American English. I then investigate an acoustic proxy for advance planning (duration of the word following is) and find that, while it does play a role in conditioning contraction, it does not interact with the following constituent effect. I connect this finding to the proposal that advance planning scope differs for different levels of grammar (Wagner et al. 2010). More broadly, I underscore that the patterning of sociolinguistic variation may be shaped, not only by the language-internal and social factors that are familiar from decades of research, but also by constraints on the language production system.