This has got to be one of the most popular questions! The following information is from Lancer(Impressions employee) originally posted in the official C3 forum. Do make sure you BACKUP the file before editing it!

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 "The C3_model.txt contains building and housing info. Its a plain text file that you can mess around with at will, though you are of course advised to keep a backup of the original in case you want to go back to using it (or in case you goof and totally hose something).

 Mess with this file at your own risk (ie. keep an original copy of it somewhere and don't bug us if you lose your game on account of messing with it: as a last resort you can always reinstall). Don't read any further unless you really want to mess around with the game model.

 Disclaimer done, for those of you who just love to mess with things:

Housing section:

Each item below is listed in the file as a single number separated by a column. The numbers represent, in order:

1. Devolve #: Desirability at which the house drops to the next lower level, be sure this is less than the next number.
2. Evolve #: Desirability at which the house evolves to next higher level. As you can see, both evolve/devolve can be negative, the top end of the scale is 100.
3. Entertainment #: Scale is 0-100, I just posted elsewhere a detailed explanation on Ent.
4. Water requirement: 1=wells, 2=fountains needed.
5. #Religions needed. Don't bother trying to bump this number to 5, I wanted to for luxury palaces just to make more of a difference between them and large palaces but couldn't due to a big hairy bug, so 4's the max (nothing blows up: the previous level house will say it needs a 5th religion, but it will evolve even if it has only 4).
6. Education requirement: 1=school or library, 2=school and library, 3=school, library and academy.
7. Market access (really food requirement, 1=they are eating).
8. Barber access, 1=needed
9. Bath access, 1=needed
Doctor/Hospital access: This was changed, much for the better, at the last moment (you don't want to know the oldmethod), I believe it is: 1=doctor or surgeon walker access (surgeons come from hospitals) 2=doctor and surgeon access.
10. #Food types eaten. We have 0-3 as the range, feel free to try a requirement of 4 if you like: it may or may not work.
11. Pottery required?
12. Oil required?
13. Furniture required?
14. Wine required?
15. Crime increment: this is the number of points the crime level will increase for that type of house every so many game ticks. So, the higher the number here, the faster the crime level in the house grows.
16. Crime base: [doesn't do anything].
17. Prosperity cap. I know you're all very interested in this one. This number, multiplied by the number of occupants in the house, summed with all of the other prop. cap #'s * occupants and divided by the total number of houses on the map is equal to the limit on how high your prosperity can go. Don't think these numbers were easy to come by, so mess with them at your own risk: if they're too low you won't be able to meet your prosperity requirements in certain scenarios. Raising these numbers makes the prosperity requirement easier to reach, and vice versa. Any number >0 (don't use 0 or negative numbers or you'll risk a divide by 0 error) will do. At one point I had 2000 for luxury palaces rather than 1750, for houses 1-12 (up to grand villas) I myself play with lower numbers for more of a challenge, tho I like a few of the numbers for patrician houses a bit higher. Email me if your curious as to what I use for personal play.
18. Population limit. The number of people in the house. Note that for housing up to the medium insulae level, this number is for a single tile building and NOT for the 2x2 (which holds 4x more people). Large insulae and beyond only have one size for each house#.
19. Tax rate: This is the tax multiplier for the house. Actually its double the tax multiplier (we needed a broader range of numbers for finer control). To find out how much a house pays in taxes,take the number of people it can hold (or, in the game, how many it currently holds), and multiply by this tax multiplier/2. This is the base tax rate, the actual taxes paid is ((base * (current tax rate in the city))/ tweak constant). The 'current tax rate' is simply a number from 0-25, not a percent, the tweak constant ended up at about 8.5.
20. (and above) None of the remaining numbers are used by the game, they're mainly the results of formulae in excell used for balancing the earlier numbers, especially the prosp cap and tax multipliers.

Building numbers:
Not everything you think you'll be able to do here will have an effect, and not all of the numbers in existence in this file have any meaning: some of it is hard-coded due to the uniqueness of certain buildings. This already has a key in the file, I'll follow it here:

• a) CST: Cost of the building.
• b) DES: Desirability. This is the initial desirability effect at range 1 (ie. the tiles surround the outer edge of the building) Can be positive or negative or zero.
• c) STP: The 'step', this is the number of tiles the desirabilty remains at a certain level before it changes (can be positive or zero, but not negative).
• d) SZE: The 'size', this is the amount of desirability change at each step (can be positive,negative, or zero).
• e) RGE: The range of the desirability effect. Ranges greater than 6 are treated as range 6 in the game. Had to make this limit for speed reasons.
• f) EMP: The number of Employees the building requires.
• I don't know what the last two numbers are, I think they're unused but wouldn't want to mess with them.

 Example Desirability effect:
 Large temples have a listed rating of 8, 2, -1, 8
 This gives a desirability effect of (8, 8, 7, 7, 6, 6)
 The effect changes every 2 tiles because the 'step' is 2, it changes by -1 each change because the size is -1. Note that the actual range in the game is 6, and not 8.

Additional notes: 2 senates and 2 forums are listed, only one of each is used in the game, you can tell by the cost of the building. I would refrain from changing the listed labor requirements for reservoirs/wells and governor's residences: things could get ugly. The cost for bridges is per tile, like roads/aqueducts and other obvious things. 'Warehouse' refers to the tile of the warehouse containing the pulley, the 'warehouse space' refers to each remaining tile (experiment if you want, but no promises). Be warned: desirability effects are per building and overlap, so even a tiny change to a single tile structure's desirability effect gets multiplied quickly due to the overlap(this is why gardens, plazas and small statues have a big effect, but be VERY careful of messing with housing desirability effects as they overlap on each other repeatedly and changes greatly effect evolution/devolution of housing). The 'Fort' row should not be touched at all, if you want to modify forts play with the 'Fort - javelin' type rows for the 3 specific types of fort.

Have fun, and try not to screw things up too bad, and don't forget to backup the original file before messing with this or trying out someone else hacked file. This is really about the only 'easy' way to hack around in the game model, most everything else is saved in a compressed format. "