Clearly, the entire property became marital as soon as stbx put OP's name on the deed.
This is the issue.
Are you sure about the resolution?
I can't seem to find support for that theory, except through the path I charted:
Of course, with the name on title an argument could be made to compare this situation with personal property where the presumption would be joint ownership by the entrieties because the property was used and intended for both and the title presumption will not help to change that. (Assuming the OP and spouse lived in the house.)
And that, took a few legal and factual leaps to get there.
Why is it important?
Other states do things differently. For example, in Kentucky, while dealing with an equitable distribution issue of whether the husband's interest in Autumn Park was marital property because it was jointly titled, the wife argued that "all separate assets should become marital property when they are titled jointly."
Appellant is asking this Court to adopt the doctrine of transmutation, which has been described as follows:
[Transmutation] occurs when separate property is treated in such a way as to give evidence of an intention that it become marital property. One method of causing transmutation is to purchase property with separate funds but to take title in joint tenancy. This may also be done by placing separate property in the names of both spouses. The rationale underlying [this doctrine] is that dealing with property in [this] way[ ] creates a rebuttable presumption of a gift to the marital estate. This presumption is based also upon the provision in many marital property statutes that property acquired during the marriage is presumed marital. The presumption can be rebutted by evidence of circumstances or communications clearly indicating an intent that the property remain separate.
Sexton, 125 S.W.3d at 270 (bracketed insertions by the court) (quoting 2 Homer H. Clark Jr., The Law of Domestic Relations in the United States 16.2 at 185 (2d ed. 1987)).
With that description, the court dismissed the argument.
We decline to adopt the doctrine because it is inconsistent "with Kentucky's property division statute, which makes title irrelevant in determining property's character[,]" with the principles of tracing established by Kentucky case law, and with Kentucky's source of funds rule. We would also note that transmutation through joint title would cause spouses not to trust each other. Rather than placing property in their joint names for estate planning purposes, they would be careful that their nonmarital property remains solely in their individual names and does not transmute into marital property. We recognize that nonmarital property can in effect transmute into marital property when a nonmarital claimant fails to adequately trace his or her nonmarital interest and overcome the marital property presumption, but that is simply a failure of proof and not the situation here. Accordingly, we reject Appellant's contention that Appellee's nonmarital interest in the apartment building transmuted into a marital interest solely because the partnership interest was placed in their joint names.
Sexton, 125 S.W.3d at 271 (footnotes omitted) (quoting L. Graham & J. Keller, Kentucky Practice, Domestic Relations Law 15.14 (2d ed. 1997)).
The problem is, I can't seem to see similar or dissimilar opinions for Pennsylvania. That is why I continue to ask if anyone has a citation that resolves the issue?
Info edit:
For my, "of course". Of course, if the spouse intended to make a gift to the OP on the titling (and not just for estate planning purposes), I suspect most any state will give marital property rights over the whole. (Except for District of Columbia.)