Question is a bit broad but i believe it's still valid for this site. A good initial site survey is very important.
Depending on the scale of this, you may need a professional networking company to run the survey for you. I will assume that this has a learning purpose so i'll list what we do when we go to a new site.
Disclaimer: a proper network survey is both physical hard work and a matter of fine details, do not take anything for granted and write down everything and take lots of visual documentation (bring a digital camera with you and abuse it).
Also, request a floor plan to the construction company if it's a new building or to the previous owner if it's an old one.
You need to check these things:
How many workers will the structure have to accomodate? Will there be temporary stations? (such as commuting managers from a different city etc.)
Is this a converged, structured cabling or a simple, lowbie installation? You can tell this by checking if there are only twisted pair, TIA/EIA-568-compliant cables or old analog phone cabling as well.
Depending on your local safety regulation, you might have to check if the network outlets share a wall box with power outlets. In many countries this is out of spec and at least in Italy you get very big fines if that is still the case when an inspection happens.
The presence of a dedicated network room and some facilities on every floor (cabinets, cable guides, under-floor cable baskets) to seat network equipment is also indication of a structured site.
What category are cables and network access points in the various rooms? A net access point is the network jack on a wall , usually close to (but not next to)a power outlet. They should be Cat.5e or Cat.6 depending if the designer meant for it to be a 100 mbps or 1 Gbit network. All the pieces of the chain need to be of a given Cat. number. If you have different cat. numbers (eg: network outlets are cat5, cables are cat6, patch panel jacks are Cat.6a) you will have to assume the whole network is rated for the lowest Number. I would strongly advise to replace anything below Cat.5e in a modern installation.
Why do you need to know the Cat. number? Because a Cat.5e is not really future proof and cannot operate at the frequencies the network of today/tomorrow will run at. It's a physical limitation due to the amount of twisting the cable received while being manufactured. Again, these are general rules for massive installations where everything needs to be according to specification. If you have 2 computers and a D-Link wifi router it doesn't really apply to you.
You need a network topology document that contains all the mapping between network access points and patch panel jacks. The floor plan really helps here because you can just create a simple Visio diagram with the floor plan superimposed.
Expanding on point 4, you need good network documentation. This is overlooked so much usually. How are you connected to the company WAN? what about the internet? Where does the carrier terminates the connections? You need to have a room or a space allocated for what is called a "demarcation point", which is where the ISP network ends and your company ones begins. This is usually a dedicated rack with a few ISP devices or a small C.P.E. (Customer Premises Equipment) box in smaller environments.
Phones. Are you going for Voip? if yes, are you gonna use a solution that allows you to work with just one network access for both the work station and the phone? If that's a negative, remember to make an account of all the workers in the building and double that to have the amount of network accesses you will need. In BIG structures, i usually 2.05-2.1x that to allow for spare stations.
If you have a dedicated network room, you need to acquire your local public safety regulations and accomodate room and budget for safety devices such as fire suppression, access restriction to the server room and so on. I've been setting up enterprise-level network rooms for 2 years now and i still contract this side of things, too easy to screw up and in Italy you need a license to install life-saving equipment.
Your network room needs AC. Again, contract this side of things but be sure you have something dependable.
How many power outlet do you have access to per seat? And for the server room (if any) how many Amps you have available for your equipment? (servers, UPS, power conditioners, networking equipment, shadow Bitcoin operation)
I'm sure the good folks at serverfault will fill in the blanks i surely left :)