The crack in the basement wall that started this
You're probably standing in the basement right now, looking at where the old stone or block foundation meets the spot where you want to build out. Maybe you noticed a hairline crack after measuring for the addition. Maybe the contractor you called for an estimate asked a question about footings you couldn't answer. Either way, something about tying new construction to a foundation that's been sitting there since the 1940s or 50s has made you pause, and that's the right instinct.
Royersford has a lot of housing stock from that era, plus older farmhouses along the Schuylkill that go back further. Fieldstone and parged block foundations are common, and they were built to carry the original footprint of the house, nothing more. Adding square footage means adding weight and shifting water flow around a foundation that was never engineered for either.
What Montgomery County winters have already done to your footings
Frost heave is the thing to think about first. This part of Pennsylvania gets a frost line deep enough that any new footing has to go down below it, typically well past three feet, or the ground will push the new structure up and down with the freeze-thaw cycle every winter. An old foundation that's been in the ground for seventy years has usually settled into equilibrium with the frost line. A new footing poured next to it hasn't, and if it's not dug to the right depth, you'll see differential movement within a year or two, showing up as cracks right at the seam where old meets new.
You can check your existing foundation's depth by looking at any exposed exterior corner where the grade has eroded a bit, or by asking whoever built the house if records survive. A contractor pulling a permit for the addition will need this number before designing the new footings, and guessing isn't an option here.
More on this from D&R Home Solutions in Royersford.
The water table and clay soil working against you
Royersford sits in a river valley with a mix of clay and shale-derived soil that doesn't drain fast. After a heavy spring rain, or the kind of nor'easter that dumps a few inches in a day, water sits longer in this soil than it would in sandier ground further out. That matters for an addition because new foundation walls change how water moves around the house. If the new footprint blocks a path water used to take around the old foundation, you can end up pushing moisture toward a basement wall that's stayed dry for decades.
Look at your yard after the next real rain. Note where water pools and where it runs off. That's useful information to hand to whoever designs the addition, because grading and drainage around the new structure need to account for it, not just the new foundation itself.
Where old foundations hide their own history
Houses of this age in the area have often had work done on them that isn't documented. A previous owner may have underpinned part of the foundation, patched a wall with a different mix of mortar, or added a addition of their own decades ago that was never inspected properly. None of this is visible from a quick walk-through.
This is where it stops being something you can assess yourself. A general contractor experienced with older Montgomery County housing will typically want to open up a section of the existing foundation, or at minimum get eyes on it from the outside at grade level, before finalizing how the new foundation ties in. This isn't overcaution. Tying new concrete to old stone or block requires knowing what's actually there, not what the original blueprints say was there, assuming you even have blueprints.
What you can reasonably do before calling someone
Walk your basement and crawlspace with a flashlight and note every crack, every damp patch, every spot where parging has fallen away. Take photos with a tape measure in frame so scale is clear. Check whether your sump pump, if you have one, runs often or rarely, since that tells you something about how saturated the soil around your foundation already is.
Pull up whatever paperwork exists on the house, even an old survey or a disclosure form from when you bought it. Any mention of foundation repair, waterproofing, or additions done by previous owners is worth flagging early rather than letting a contractor discover it mid-project.
Once you've done that groundwork, the next step belongs to a general contractor who can evaluate the existing foundation's condition, coordinate with whoever does the structural and soil work, and plan the new footings against both the frost line and the drainage patterns specific to your lot. Trying to design that connection yourself, or trusting a quick visual estimate, is how additions end up with the cracked seam and the damp corner that shows up two winters later.