My builder upcharges to lay tile diagonal versus straight. I am not sure if this requires more labor but he says it does."

Every tile at the edge must be cut.

'Old work' is just about always more time consuming than new work.

You are also left with wall repairs on old work.

The wring is installed before the walls are closed up, so the 'repairs' consist of cutting holes in the wall surface as it is installed.
Better price out the whole job (wiring, wall repairs, painting, etc.) before deciding.

It is very difficult to get code compliant outlets without mangling the walls. Everything you do later may mess up insulation in exterior walls.

As the market got tight, the incentive to build a base at a lower price and increase the upcharges increased. It is a common scenario.

Upcharges by tract builders have always been a profit line, it just seems they have become more of a profit line.

Diagonal tile is always more expensive but I think our upcharge for a room was $50 for a basic bathroom. The labor charge was about 10% higher.

Are you doing a custom build or working with a tract builder?

Also, you may want to be specific regarding what the up charges are so that others on here can help you figure out if they are reasonable.

"My builder upcharges to lay tile diagonal versus straight. I am not sure if this requires more labor but he says it does." This requires the builder to purchase more tile (because there will be more waste), to cut every single edge tile, and to basically do more "math" type thinking.

My husband and I laid our own tile in our bathrooms/laundry room a few years ago, but hired a professional to do the kitchen on a diagonal. The added expense was SO worth it!