We will be getting someone to build a 25m² veranda, just a basic one like 4 posts (or 2 posts if it can safely hookup with existing roofing) and flat roof.

I wonder what are the pros and cons of timber and steel verandas?

Also, I thought colourbond steel is the standard staff, but the guy said it's not, there
s only zinc plated steel or alloy, and alloy will cost a lot more.

My personal choice would be to use timber, it really depends on who you are getting to build it.

Timber;
Pros; easy to work with, no special tools required, can do it yourself.
Cons; some maintenance required such as painting every few years, not as strong as steel.

Steel;
Pros; high strength, dead straight, low maintenance
Cons; may need special equipment such as cut off saw, welder and grinder, generally harder to work with, especialy if your slab is not perfectly level or your roof isn't perfectly square.

You can definitely get Colourbond steel, from Stratco or Stramit. That's what we have, and have been happy with the steel structure with timber decking, 5 years old now. As we are in a termite and bushfire area, steel was a no-brainer.

The new deck is steel and Hebel, due to new bushfire regulations; not as nice, but we could not use any wood.

Yeah, thanks! We have decided on steel as well.

To join the veranda to the house, the options are:

1) Have it aligned with (at the same height) of the rain gutter, that is existing roof > existing rain gutter > new veranda roof.

2) The alternative is to have the new roof under the existing gutter. It will lose may be 10cm of head room, but I guess the new roof can catch any overflowing rain water if that ever happens.

Is any one option better than the order?

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Or what we did was have the roof slope up away from the house.

They use steel posts through 2 tiles for a 5m span, and a boot to seal against the tile.

No gutters necessary as it drains back to the main roof, increased height, and allows a breeze to cool the area. From inside, does not look like you are living in a cave, as opens up the view rather than closing it down.

We thought of that as well, I think a major benefit is that there is an escape for wind pressure (the breeze).

The issue is that we aren't comfortable with the installer touching the roof, we need a 5.5m x 5m roof area, he mentioned 2 posts through 2 tiles into the beam, and than 3 posts into the ground at the front. He will use silicone to seal up the broken tiles, no mentioning of boots.

Roughly how much did they quoted for your verandah? These guys are asking $100/m², does it sound right? I need to watch out if it's too cheap. But they installed one at my parents house and workmanship seems ok.

Personally i would go steel.

and i would align it under the gutter. i only say that because at our old house we had a large pitched pergola, and if we had anything heavier than a shower the gutters couldn't cope with two roofs coming into one gutter, so if you align it underneath, it will catch gutter overflow, and send it to the gutter on the opposite side

You definitely need the boot; we have it for our deck/balcony (well, it's a deck which happens to be also 2.5m off the ground at the highest), as well as the tubing for the electric cable for our roof solar panel install. And it was also with plenty of silicon around. I was impressed that they managed to drill the tiles without breaking them, but we have concrete, which the solar guys apparently like as they are less likely to break.

What he proposed sounds about right; there is less wind pressure, so you don't have the problem of having great strength. Ours is 5 x 2m, and uses two post through the tiles, and two posts onto the ground for the roof (more for the deck of course).

We paid a lot for ours, probably way over the odds as we later found out. It was $400/m2 at the time for the roofing alone, same again for the hardwood deck with steel framing, plus extra for the rails; admittedly it was up to 2.5m off the ground in parts and we are in Sydney.