Do you need the money right now or not? Can you afford to build a home on spec for that price? Can you afford to sit on the home for a few years if the interest rates go up while paying HOA fees if there are some, taxes, upkeep on the home etc? How about building costs? Lumber and other building commodities skyrocketed during Covid raising building costs to the point contractors raised their price to finish or just paid out a non completion penalty as it was less than the material inflation costs and walked off. Can you afford to dump $100k into a house then be left scrambling for someone else to complete it at the new materials cost? Lot's of people do it but only you can decide if you can afford it. Budget it out and include the average sale time of the homes in the community with what it would cost you to keep the home that long plus a buffer in worst case scenario. Building a spec house is a good way to make money but most of the guys I know doing it are contractors themselves so are doing it for cost and can sell cheaper for the same profit others do plus the cash flow to sit on it if they have to.
Say the average time on the market in your community is 2 years and the selling price has to be discounted for the property to move. Average lowering price is $25,000 before sale. It costs you $15,000 a year in taxes, maintenance etc. That $200,000 home you put on the market for $300k just required you to drop it to $275k to move it. $30k plus 6% brokerage fee to sell is $46,500 plus other nonsense so say $47,500. You sat on a house for 2 yrs and made $25,000 which is a good profit but what is the red hot real estate market doing in 2 years? Will your house sell quickly or will you have to wait 3 or more years? Will house prices drop to keep them moving? Did you have to borrow equity in your home or from a 401k to get the deal done? What if another emergency pops up and now you don't have the equity in anything to pay for it? What if your cash flow from your job stops because you broke your leg and can't work for 2 months. Do you have an emergency fund to draw on to keep the bills paid? Don't believe a real estate broker on any of this, they will paint you a happy picture to get your listing.
You only said you paid "a song and a dance" for the lot. If you paid $8,000 for it and sold it for $16,000 that's a 100% profit. Not good enough? It would be for me and I'd take the $16,000, look around and do it again and again and again. Pretty soon your buying $100k property cash and selling for $150 or $200 without risk as you pulled your original $8,000 out long ago, allocated 5 or 10% of the profit to yourself as a "salary" and are now playing with other peoples money. If you lose everything then you still have a nice profit plus your original money in the bank.