To present a plan or not...that is the question.
Most homeowner's have no idea what they are looking at on an irrigation design when it's put on the table in front of them. They see the lines, the dots, and all the other symbols used to designate the various components. And then when two or three or more contractors follow with their design, total confusion results and they generally go with the company that did the best job selling themselves and their services, or the lowest price.
Sometimes with a design on paper, the homeowner may wonder why there are not the same exact number of heads in the yard as there were on the drawing. There may be extenuating circumstances and field changes that make for a more efficient system that may result in fewer heads than designed for. Then you run the risk of having to justify why there are only 65 heads in the yard and your design had shown 73. And then some will want a credit for the unused heads. But on the other hand, if you made a field change that required 3 or 4 more heads, they damn sure don't want to hear about paying for the extra heads. A simple way to alleviate this is to have verbage in the contract stating that "The system will provide complete coverage to all irrigated areas specified in this contract per discussions between the owner and contractor." Then you have covered all the bases.
If I run into a potential customer that absolutely has to have a drawing before signing the contract, I will charge them accordingly - no less than $150.00 for an average sized lot, and the price goes up from there depending on the complexity of the project. If they then choose to go with us, that design fee is deducted from the final payment. (Well some of it is anyway.) While some may argue that it is a cost of doing business in the irrigation world, that time spent doing a full-blown design has to be paid for somehow, and it damn sure isn't coming out of my pocket!
As Bryan said, I provide an "as-built" drawing if requested. It shows the location of the P.O.C., the backflow device, the mainline path, wire path, valve locations, main shut-off, and the controller location. This will provide them with any necessary information that they need for working around the yard without damaging any of the major system components. Lateral lines are not on the 'as-built' nor are heads. Some might feel it is necessary, but it is just as easy to turn on the system and find the heads when popped up.
When I did designs as part of the bidding process, I quickly learned that it is never prudent to leave a drawing with the customer, for the reasons already mentioned. It really bites to drive past a job that you didn't get, and see the installing contractor using your design to install the system. That happened only once. I used to take my original drawing on vellum during the show-and-tell. That way if they asked to keep it to 'study' my stock response was that "this is my original drawing and I don't have any other copies. If something happened to it, it will have to be redrawn at an additional expense." Then I would ask them if they felt that they would understand the system any better from looking at it later when I wasn't there to answer any questions they might have.
I talked to many contractors over the years and they all agreed that it cost money to give a free estimate - probably in the neighborhood of $50.00 - $75.00 if they took the time to measure the property, go back to the office and draw up a proposal and then return to try and close the sale. Throw in a couple hours for doing a design, and pretty soon that 'free estimate' has cost you, the contractor, about $300.00 or more. Sure if you have a CAD program you can whip out a design in short order, but that CAD software cost you money as did the printer and the other hardware necessary to use it.
If I can't sell an irrigation system without a design, then I must be doing something wrong. A good presentation folder with cut sheets for the equipment, and professional looking proposal - free of spelling and grammatical mistakes, and a damn good gift of gab will get just as many jobs.
Just my thoughts,
Jerry R