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Read the new work against the catalogo concepto by concepto and against what each existing unit price already includes in its specification. Write down, in one paragraph, which conceptos were considered and why none of them reaches this work, and define the scope and the unit of the new concepto precisely enough that two people would measure it the same way.

لماذا: The first question the reviewer asks is whether an existing concepto already pays for this, and a contractor who cannot answer it in writing loses the price whatever the analysis behind it looks like. Work that is genuinely inside an existing unit price and gets paid again as extraordinary is the observation that comes back at audit years later, with the money already spent.

Have the residente de obra record the instruction in the bitacora as a numbered note that says what is to be executed and why, and answer it in the bitacora too. Keep the note number with the new concepto from this point on, so every later document, analysis, estimacion and generador carries the same reference.

لماذا: The bitacora is the instrument of communication the contract recognises, so an instruction that lives anywhere else has no date and, in practice, no author. The note number is also what lets the price, the volume and the payment be tied together by a reviewer who was not there, and a chain that cannot be followed is one that gets unwound.

Build the costos basicos of the new concepto from the materials, labour, machinery and auxiliary costs already analysed in the winning proposal, at the values that proposal carried. Only where an input does not exist there do you go to the market, and then you attach the quotations that justify it and say so on the face of the analysis.

لماذا: The rule is that the extraordinary price is built on the same footing as the contract, because a contractor who may reprice its own inputs mid-contract holds a lever no tender ever put in the price. Marking the genuinely new inputs is what keeps the review on those three or four lines instead of on the whole analysis.

Apply the indirectos, then the financiamiento, then the utilidad and finally the cargos adicionales, each at the percentage the winning proposal carried and each computed on the base the previous step produced rather than all of them on the direct cost. Present the analysis in the same layout the proposal used.

لماذا: The order is fixed, and a cascade computed flat on the direct cost gives a different answer that looks reasonable and is wrong, in the contractor's favour often enough that reviewers check it first. Presenting it in the proposal's own layout means the reviewer compares like with like, which is the difference between a price agreed in one round and one agreed in four.

Submit the analysis to the dependencia, work through its observations line by line rather than by resubmitting a different total, and record the price it authorises. Where the new work also moves the contract amount or the period beyond what the contract already allows, formalise it in a convenio modificatorio and check the change against the ceiling the law puts on modifying a contract at all.

لماذا: An authorised price is what the estimacion can be built on and an unauthorised one is work already executed at the contractor's risk. The ceiling matters here rather than later: a change that exceeds what the law permits cannot be rescued by a convenio, it has to become its own procurement, and finding that out after the work is built leaves the money with no lawful route to be paid.

Add the concepto to the estimacion at the authorised price with its own numero generador showing how the volume was measured, keeping the extraordinary conceptos identifiable rather than mixed into the original catalogo lines. Amortise the anticipo and hold the fondo de garantia on them exactly as on the rest.

لماذا: Extraordinary conceptos are the first thing an auditor separates out, so an estimacion that separates them already answers the question. Keeping the amortisation and the retention rules identical avoids the other common finding, which is a concepto paid in full while the rest of the contract was still funding an advance.

Put the period for presenting the analysis and the period for the dependencia to resolve it under a reminder, counted in natural days from the date in the bitacora, and set the warning far enough ahead that the analysis is finished rather than started when it fires.

لماذا: Natural days include weekends and holidays, so a period tracked as working days is always longer than the real one and runs out earlier than the calendar suggests. A price presented late is argued about on its lateness rather than on its arithmetic, and the work is usually already built by then.