Make architecture in minecraft

What can we use if we don't have the block types we picked for walls, floor, or roof?

If you can't find a chosen block from the 'Choose 4 to 6 block types' step, substitute visually similar blocks (for example stone bricks or smooth stone instead of quartz, plain glass for stained glass, and glowstone or torches instead of sea lanterns) and use the same substitutions on both sides so the symmetry stays consistent.

How can we fix the building if the two sides don't match while building walls and windows?

Follow the 'Measure the footprint...mark the building corners' step and count blocks from the symmetry axis, place temporary marker blocks along that line, then stand on the symmetry line to compare left and right and remove or add blocks until every wall block, door, and window has a matching block on the opposite side.

How should we change the activity for younger or older kids?

For younger kids simplify the blueprint to a small footprint, use only 2–3 block types and help them mark corners and place doors, while older kids can design larger blueprints, choose 4–6 detailed block types, add mirrored interiors and symmetrical redstone mechanisms as part of 'Add lighting stairs and decorative details.'

What are some fun ways to personalize or extend our symmetric Minecraft build before sharing it?

Extend the project by adding mirrored outside features like a matching garden, moat, or twin towers, build mirrored secret rooms or redstone lighting that obeys the 'left and right sides mirror each other' rule, and take screenshots plus your blueprint to share on DIY.org.