ASK MOST people what it cost to remodel their house, and they'll tell you "too much." Now and then, though, some reasonably honest soul will tell you the truth: They don't know. In most cases, they can't bear to think about how much. Not so Joyce Deroy. She'll tell right off, the remodeling of her Dupont Circle (Swann Street) townhouse cost $32,383.01.

Deroy is, by profession, a designer of computer systems for a legal group. As such, she has access to a computer system - sometimes, indeed, she even has a computer terminal at home, when she has to work late. So it seemed natural for this mathematical mind to keep the whole remodeling process on computer tape, from how much all the paint cost to a room-by-room survey of expenses.

(The Deroy house and the computer printout of costs - along with 12 houses in various degrees of restoration in the 19th Street area form Dupont Circle north to Florida Avenue - will open from 1 to 5 p.m. today during the 10th annual Dupont Circle Home Tour. Tickets are $7 at 1921 S St. NW.)

The amount of money one can spend on small, insignificant but necessary things really hits you when you take a look through Deroy's list. Consider a few:

Paint, $49.82; paint brush, $1.68; putty knife, $2.51; keys for roffer, floor man, electrician and tiler, $3.57; paste wax for wood floors, $5.67; sealer for kitchen floor, $31.50; drill, wire brush for plaster removal, $17.61; buckets for muraitic acid, $2.06; and, again, paint brush, $4.35; and a cold chisel masonry hammer for plaster removal, $17.78 plus a wire brush for ditto, $3.15; putty knife, spackle, plastic, dustpan, gloves (disposable), $11.01.

Then there are the services that you'd never think about: to Phillip Dickens, $264 to bleed the raidators, repair leak in the heating system, drains, etc.; clean, adjust oven and rebolt oven door hinge, $23.63; install faucet on bathroom basin, $53.34.

Sometimes the big services don't seem half bad in comparison to the items that "nicked and dime you to death."

Gordon McDaniel carpentry contracting (mostly labor, some materials). $13.384.53; Binky Construction Company, install kitchen floor and three fireplaces, $2,383,73; Russell Spittle, paint/patch main roof and lower roof. $305; doors and installation, $144.83; Fred Johnson, removal of concrete slabs and other trash. $225; repair front door, $108.51. M&M Drywall Co., drywall and plastering work for entire house, $1.450; Joseph Kelly, install ceramic tile. $490. Gordon Bromfield and Associates, fence and deck work, $1.550.

Some things, in retrospect, seem positively cheap: skylight, $265; Ademas tile for the kitchen floor, $632.94; Kitchen trck lighting $59.63; total electrical fixtures. Maurice Electric Co., $1,135.66.

And then there are the ones you grit your teeth at: blinds for the third-floor bedroom, $257.85, not as nice as the ones from Bloomingdale's at $20.80 for the second floor, and certainly not as handsome as the wooden shutters for the front room, first floor at $50.30.

Joyce Deroy found out other things as she went along. "At first I tried to do all the remodeling out of income, but after a year or so of that, I found out about remodeling loans. That made the work go a great deal faster."

Deroy paid $36,000 for the house 21/2 years ago. After she'd been its sole support for about a year, she married Wilie King, who had a fine, already restored house on Capitol Hill. They lived in his house while working on hers.

"We hired for most of it," said Deroy. "I found most of skills were destructive, like tearing things out and stripping the paint."

Deroy and King did most of their own design for the house, with help from the craftsmen, such as Joseph Kelly, who designed as well as built their handsome circular fireplace opening.

They use the front room, the orginal parlor, as their sitting room. The middle room is the dining room, and the kitchen, at back, overlooks a pleasant backyard and deck. The fence was built by Jim King, Willie's brother, who has since set himself up as a carpenter. The living room is sensibly on the second-floor level, away from the noise and pryig eyes of the street. Two smaller rooms here serve as den and guest bedroom. The master bedroom is far above the street on the third floor.

Now that it's all finished, Deroy and King have bought a house on a point of land in the Patuxent River, and are starting all over again, writing down every penny as they go, of course.

Another way to go is the way Peter and Ellen Acly and David Jones remodeled their house on 19th Street. They bought a row house together three years ago as tenants-in-common. The house was originally the proverbial rabbit warren of apartments. Architect Jones designed a floor-by-floor remodeling.

His apartment is quite modern, with all sorts of archtect's tricks, such as curved walls and beams to extend the lines of the bay. The Aclys' remodeling emphasizes the Victorian period of the house, with a great mantlepiece stripped down to the original oak. The two rental apartments were just cleaned up and repainted.

The house cost them $67,000, as was. The was was terrible. "My floor, the top one, was full of water. An acoustical title ceiling had been thrown up at one time, and it was falling down in great wet gobs," Jones said.

Jones and Peter Acly did the demolition and painting. Jones also took on some particular jobs that the contractor wasn't prepared to tackle - making the curve for instance, which forms the walls of the fireplace, Jones managed that by soaking the wallboard in the shower and forming it while it was wet. He did another curve for a baseboard using the same technique with wood.

Jones' apartment offers some interesting design ideas for other remodelers. Because the room was 17 by 20, not very large, he separated living and dinning with an L-shaped sofa plus the fireplace wall. He built the sofa himself and had the 12 pillows (back and seat) made from pseudo-suede.

The dining table is a butcher block he bought several years ago from an officer's club counter. It is put together with rods for strength and port-ability. In this apartment, he supports it on one side from angles set into the brick wall and on the other by a column that goes to the ceiling.

A tall, fluted architectural remnant column from Artifacts in Alexandria serves to hold plants and act as point of interest.

Jones figures the entire house remodeling, including fixing the roof and painting the halls, cost about $25,000, or about $12 a square foot.

Oh, yes, Jones was married last weekend and his bride insisted they move to Georgetown.