An addition is new walls attached to an old house. The new part gets built to today’s code. The old part was built to whatever the code said when the house went up, which might have been decades ago. The siding has to make the two look like one house, and keep water out of the seam between them.

That seam deserves most of your planning attention.

New walls, old walls

Since 2006, the BC Building Code has required a rainscreen on new homes in coastal BC. That’s a drained gap, usually made with vertical strapping, between the siding and the wall behind it. Your addition’s new walls will likely get one. If your house went up before then, its walls may not have one.

The strapping moves the face of the new siding out from the wall. So where the new siding meets the old, the two surfaces won’t sit flush unless someone plans for it.

The joint is the risky part

Water problems on additions tend to show up where new meets old. Settle these early:

  • How the new wall’s drainage gap ends where it meets the old wall, and where that water gets out.
  • How the addition’s roofline is flashed against the old wall.
  • Whether the trim and corner boards will match the old house, or set the addition apart on purpose.

These are cheap decisions while they’re still on paper, but they get expensive once the walls are closed in and the siding is up. It helps to have the siding installer look at both walls before anything is ordered. If you’re in Surrey or nearby, the team at Mi Casa Exteriors installs siding on new homes and additions, as well as re-siding older ones. Its crews are in-house, and there’s no deposit to get started.

Matching the look

Old siding fades after years of sun and rain, so a new board in the same colour won’t always match. Some owners re-side one or two old walls so the change happens at a corner, not in the middle of a wall. Others pick a different material for the addition and make the contrast part of the design.

Check how big the project really is

An addition can also change what the rest of the house has to meet. White Rock is a good example. The City of White Rock’s renovation permit rules look at the value of the work. When renovations or additions are worth more than 50% of the building’s current assessed value, the whole structure has to be brought into substantial compliance with the Building Code and city bylaws.

That can turn a simple addition into a much bigger job. Other cities have their own rules, so ask your building department early, before the plans are final.

Plan it with the framing

Siding is often the last decision on an addition, made after the framing is already standing. That’s backwards. It’s much easier to build the drainage gap and the flashing right the first time than to fix a leaking seam later.